NEW YORK (AP) _ Aniello Dellacroce, reputed underboss of the nation's largest Mafia family for almost two decades and the subject of three indictments in the last 13 months, died peacefully in a hospital of natural causes, his attorney said Tuesday.

Dellacroce used the name ''Timothy O'Neill'' - O'Neill was his knickname - to check into Mary Immaculate Hospital in the borough of Queens, where he died late Monday night.

The 71-year-old Dellacroce had been suffering from cancer and heart disease, according to his attorney, Barry Slotnick.

In April, Dellacroce was arraigned on federal racketeering charges over the telephone after he said he was too sick to travel to the federal courthouse. Slotnick described his client at the time as ''a very, very sick man, an old man who may never attend the court proceedings.''

Since the late 1960s federal and local investigators have identified Dellacroce as No. 2 man in the crime network once led by the late Carlo Gambino.

The Gambino family, which in recent years has exercised increased influence over the city's four weaker Mafia families, is said to have about 250 members and 550 criminal associates.

Dellacroce was one of 11 reputed Mafia leaders indicted in April on racketeering and conspiracy charges in connection with alleged membership in a ''commission'' that settles disputes and determines relationships in New York's Italian-American underworld. He was scheduled to go on trial in March.

Last March, Dellacroce and several other alleged members of the Gambino family - including his 29-year-old son, Armond - were indicted on federal racketeering charges. The indictment accused Dellacroce, in his capacity as underboss, of supervising Gambino members involved in loansharking, murder and hijacking over an 18-year period.

In November 1984 he was indicted on charges of evading federal taxes.

According to law enforcers, Dellacroce had a long, violent and profitable career in organized crime that dated to the 1950s, when he was associated with Albert Anastasia and his casino interests in pre-Castro Cuba.

In 1966 he was one of 13 alleged mobsters arrested at the ''Little Apalachin'' meeting of mobsters at a restaurant in Queens. Also attending were Carlo Gambino, Santo Trafficante Jr. of Florida and Carlos Marcello of New Orleans.

His base of operations was the Ravente Social Club in Manhattan's Little Italy, where police said he would broker deals and arbitrate disputes.

Federal Judge Arnold Bauman, in rejecting bail for Dellacroce during his appeal on a tax charge conviction in 1973, called him ''a top level hoodlum, a danger to society, a menace to the community, a parasite who lives off the life blood of honest people.''

Six years later he was described in court as ''the godfather of all crime in New York City'' by a Manhattan assistant district attorney.

But Dellacroce, whose name means ''Little Lamb of the Cross,'' spent only six years in prison.

Although Dellacroce appeared to investigators to be acting boss of the family following Gambino's death in 1976, by 1978 FBI sources were reporting that Paul Castellano had been named family boss.

But in 1980, the newspaper Newsday quoted a federal law enforcement source as saying that Castellano ''doesn't call the shots that count. The man that does is Dellacroce. He's got the firepower, the army of enforcers that keeps the family in line.''

A 1983 report by the New Jersey state police intelligence bureau referred to Dellacroce's traditional ''proclivity toward violence'' and accused him of ordering the 1979 murder of Carmine Galente, then a leader of the Bonnano crime family.

Dellacroce and other Gambino family members also were believed by authorities to have sanctioned the 1980 murder of Angelo Bruno, the head of the Mafia family in Philadelphia.