BOSTON (AP) _ Many baby deaths attributed to the mysterious sudden infant death syndrome actually result from suffocation, overbundling and other accidents that are caused by parents' poor judgment, a study concludes.

But a leading SIDS researcher cautioned that the study looked at cases that may not be typical of such deaths nationwide and said most babies who die from the syndrome show no signs of parental negligence.

The doctors investigated 26 instances of sudden infant death in New York City and found at least some evidence of accidential causes in all but two of them.

The cases are ''probably characteristic of the problem nationwide,'' said Dr. Millard Bass. ''It appears there are a lot of misdiagnoses being made.''

His investigation turned up cases in which mothers apparently smothered their babies by rolling on them in their sleep. Babies also appeared to die from being placed too close to radiators at night or were asphyxiated by their bedding.

The study was based on an investigation of deaths of babies brought Kings County Hospital Center in New York City. It was directed by Bass, a forensic pathologist at the State University of New York, and published in Thursday's New England Journal of Medicine.

In an accompanying editorial, Dr. Bradley T. Thach of St. Louis Children's Hospital said the cases might not be typical of crib deaths elsewhere, so ''considerable caution is needed in making generalizations based on these data.''

Dr. Frederick Mandell, vice president of the National Sudden Infant Death Syndrome Foundation and senior associate physician at Children's Hospital in Boston, said that although he had not seen the unreleased study he feared its effect.

''The impact of this kind of article on parents whose children have suddenly and unexpectedly died - and who have not asphyxiated their child and whose children have not died of hyperthermia - will be of great significance because all parents feel some guilt, that they have done something wrong,'' he said.

''In the grief reaction there are a lot of what onlys - if only we had not left the blanket in crib,'' he said. ''But in fact it seems most of the babies who die show no evidence of negligence.''

Mandell said previous studies refuted the findings of the Bass study.

Gayle Lloyd, spokeswoman for the national Centers for Disease Control in Atlanta, said it is their general policy not to comment on studies before their researchers have seen them.

When a seemingly healthy baby dies unexpectedly or without explanation, the cause of death is frequently attributed to sudden infant death syndrome, also known as SIDS or crib death. It is the most common cause of death among children between the first month and first year of life.

SIDS is listed as the cause of death of an estimated 8,000 to 10,000 babies in the United States each year. Although some cases are thought to result from breathing abnormalities, the cause of SIDS is largely unknown.

The latest research suggests that many such deaths actually result from preventable accidents in the home. Among the cases investigated, the researchers wrote, ''poor judgment by the caretaker of the infant was considered an important contributing factor in almost all the deaths.''

Most of the babies died in crowded tenements in the poor neighborhoods around the hospital.

Bass said that while SIDS can happen in middle-class families, victims are almost always poor. In the neighborhoods where the study was conducted, for instance, there are more than four SIDS deaths for every 1,000 births. This is 10 times higher than the rate in more affluent parts of New York City.

Bass and colleagues investigated 26 consecutive deaths in which sudden infant death syndrome was the presumed cause of death. After talking to the victims' families in their homes and examining the beds where the babies died, they concluded that there was strong evidence of accidental causes in six cases. In 18 others, there was circumstantial but less clear-cut evidence that accidents had occurred.

Among the cases they cited:

- A 3-month-old boy suffering from a cold slept in a wooden drawer on a basement floor. The child died - probably of asphyxiation and high body temperature - when the parents tried to treat him with a crude mist tent. They covered the drawer with a plastic bag and attached a vaporizer to it.

- A month-old girl was placed face down on a large foam-rummer pillow in a bassinet. The baby was found dead, trapped in a groove formed between the edge of the pillow and the plastic side of the bssinet.

- A 4-month-old boy was dressed in an undershirt and jumpsuit, covered with three blankets and put to bed in a crib next to a leaky steam radiator. The baby was found dead the next morning. Blankets covered his head, and the temperature in the crib was 105.

A study published two years ago in the British journal Lancet found that keeping babies too warm appeared to be a cause of crib death in England.