The study found that approximately one-third (35.5 percent) of the runaways met lifetime criteria for PTSD, and 16.1 percent met 12-month criteria for this disorder. Just over 90 percent of the adolescents (ages 16-19) with PTSD also met criteria for at least one of the other four diagnoses (major depressive disorder, conduct disorder, alcohol abuse, and drug abuse). More than half of the male runaways (59.1 percent) who met criteria for PTSD also met criteria for alcohol and drug abuse. Approximately half of the female runaways (48 percent) with PTSD also met criteria for alcohol abuse, and 44 percent met criteria for drug abuse. Characteristics associated with having PTSD were being older, being female, having experienced serious physical abuse and/or sexual abuse from an adult caretaker, and having been assaulted or injured with a weapon when on the street. The interaction between sexual abuse by a caretaker and sexual assault after they had run away from home was statistically significant, indicating that rape victims were likely to meet criteria for PTSD regardless of whether they had experienced early sexual abuse. The high number of PTSD adolescents who are abusing alcohol and drugs warrants serious clinical attention in treatment programs. Study data were collected by full-time street interviewers on the streets and in shelters in eight midwestern cities. Various instruments were incorporated in the interviews to assess for the presence of PTSD and the other four diagnoses. 4 tables, 1 figure, and 38 references