The stability of living situation was examined as a predictor of young people's HIV-related sexual and drug use acts two years after leaving home for the first time. Newly homeless youth aged 12-20 years were recruited in Los Angeles County, California, U.S.A. (n = 261) and Melbourne, Australia (n = 165) and followed longitudinally at 3, 6, 12, 18, and 24 months. Their family history of moves and the type and frequency of moves over the two years following becoming newly homeless were examined. Regression analyses indicated that recent sexual risk two years after becoming newly homeless was not related to the instability of youths' living situations; condom use was higher among youth with more placements in institutional settings and among males. Drug use was significantly related to having moved more often over two years and Melbourne youth used drugs significantly more than youth in Los Angeles. (Authors)