A high-powered Wall Street executive accidentally fell to his death from a window in his fifth-floor Upper East Side apartment last night, a law-enforcement source said.

Keith Mastronardi, 31, was leaning out the window while smoking when he slipped at 10:25, according to the source.

He and his wife had three children, including a baby, who were all in the East 74th Street apartment apartment at the time.

The couple had been drinking at the time when Mastronardi lost his balance and tumbled, sources said.

The woman was taken out on a stretcher, but was not injured.

Mastronardi was head of exotic derivatives at the India-based firm Vyapar Capital Market Partners.