OBJECTIVE:To estimate the prevalence of HIV infections, as well as hepatitis B and C and syphilis viruses in homeless people. METHODS:Cross-sectional study with educational intervention, conducted in the city of São Paulo, between 2002 and 2003. A convenience sample of homeless people who used night shelters was selected, according to the following criteria: aged 18 or older and not showing psychiatric disturbances. During interviews, sociodemographic and behavioral data were gathered and HIV, hepatitis B and C and syphilis laboratorial tests and post-test counseling were carried out. RESULTS:A total of 330 shelter users participated, with an average age of 40.2 years, 80.9% of them male, having lived on the streets for one year in average. Prevalences of 1.8% for HIV, 8.5% for hepatitis C virus, 30.6% for previous hepatitis B infection, 3.3% for chronic or acute infection by hepatitis B virus, and 5.7% for syphilis. The consistent use of condoms was referred to by 21.3% of interviewees and the use of injecting drugs by 3% of them. Positivity was 10% for HIV and 50% for hepatitis C virus among injectable drug users, versus 1.5% for HIV and 7.3% for hepatitis C among the others, showing an association between the virus and the use of injecting drugs. Previous imprisonment was referred to by 7.9% of women and 26.6% of men, with a prevalence of 2.6% for HIV and 17.1% for hepatitis C virus. CONCLUSIONS:The high prevalences of HIV and hepatitis B and C viruses require prevention programs based on vaccination against hepatitis B, early diagnosis of these infections and placement of homeless people into health services.