We examined the social network composition among newly homeless youth over time and assessed how pro-social and problematic peers affected sexual and drug-using HIV/AIDS risk-behaviours among 183 youth in Los Angeles County, California. The percentage of newly homeless youth who reported that 'most' or 'all' of their friends were attending school, had jobs, and got along with their families was 73%, 24%, and 50% respectively. Logistic regression models indicated that presence of these pro-social peers reduced HIV risk behaviours at two years; odds of HIV-risk were lower with a greater number of peers who attend school, have a job, or have positive family relationships or if networks change over time to include more of these peers. Presence of problematic peers increased the likelihood of HIV risk-taking; odds of HIV risk behaviours increased with a greater number of peers at baseline who steal, have overdosed, have been arrested, or are in a gang, or if networks change to include more of these peers. Interventions should target newly homeless youth in networks that contain problematic peers, but should strive to harness the naturally occurring pro-social peer influences present in these networks.