He was commander of the resident crew aboard the Russian space station Mir from February to September 1996, during which time he made six space walks and conducted many research experiments. He was commander of the fourth resident crew of the International Space Station, staying from 5 December 2001 to 19 June 2002. He has logged 389 days in space and taken eight space walks. Onufrienko was selected as a cosmonaut in 1989.