This article presents lifetime and 12-month prevalence rates and comorbidity for posttraumatic stress disorder (PTSD) among a sample of 428 homeless and runaway adolescents. Data are from baseline interviews of a longitudinal diagnostic study of 428 (187 males; 241 females) homeless and runaway adolescents aged 16-19 years (mean age = 17.4 years, SD = 1.05). The data were collected by full-time street interviewers on the streets and in shelters in eight Midwestern cities of various populations. About one-third (35.5%) of the runaways met lifetime criteria for PTSD and 16.1% met 12-month criteria for the disorder. More than 90% of the adolescents who met criteria for PTSD met criteria for at least one of the other four diagnoses. Multivariate analyses indicated that correlates of PTSD were age of adolescent, being female, having experienced serious physical abuse and/or sexual abuse from an adult caretaker, and having been assaulted or injured by weapon when on the street. The multiplicative interaction between sexual abuse by caretaker and sexual assault when the adolescents were on their own was statistically significant, indicating that rape victims were highly likely to meet criteria for PTSD regardless of early sexual abuse. At very high levels of early sexual abuse, the probability of meeting criteria for PTSD converges with that for sexual assault victims.