Joseph F. O'Brien, an Arlington man convicted of first-degree murder in the June 1984 stabbing death of a neighbor, was sentenced yesterday to 50 years in prison.

Arlington Circuit Court Judge William L. Winston imposed the sentence recommended by the jury that found O'Brien guilty in April of first-degree murder in the death of Betty Jane Konopka, 55.

O'Brien, asked in court if he had anything to say before sentencing, replied, "Yes. I'd like to state that I am innocent and I wish you'd take that into consideration in sentencing."

O'Brien, 27, has twice served time in prison for grand larceny and unauthorized use of a vehicle and will be eligible for parole in 25 years, according to Commonwealth's Attorney Henry E. Hudson.

Konopka, a secretary in the Labor Department's office of Mine Safety and Health Administration, was found dead in her house at 130 S. Woodrow St. early in the morning June 6, 1984. She had been stabbed 19 times.

A glass pane in the back door had been smashed, and her wallet and keys were missing. O'Brien was found not guilty of burglary and robbery in the incident.

John Youngs, one of O'Brien's two court-appointed attorneys, asked Winston to suspend part of the sentence, arguing that O'Brien could overcome the problems of child abuse and alcoholism that marred his past.

"Mr. O'Brien has a problem past . . . but he could in fact go on to a program that could deal with these problems and go on and be a productive citizen," Youngs said.

Hudson repeated details of the case, describing it as "a very brutal homicide," and asked Winston to impose the entire 50-year sentence.

Youngs and Brendan Feeley, the other defense attorney, said they planned to appeal the case to the Virginia Intermediate Court of Appeals.