The reliability of self-reports of sexual behaviors related to HIV transmission was examined in a study of homeless men with severe mental illness. Thirty-nine patients of a New York City shelter psychiatric program were interviewed about their sexual behaviors in the past six months. The same interview was administered twice, with a one- to two-week interval between interviews. Test-retest reliability was assessed using kappa and intraclass correlation coefficients. Reliability estimates ranged from .49 to .93 for overall sexual activity, number of partners, and specific behaviors other than receptive anal sex. Reliability was lower for condom use. The authors conclude that reliable self-reports about sexual behavior can be obtained from homeless men with severe mental illness.