Research on posttraumatic stress disorder (PTSD) among youth has focused on specific subgroups from developed countries. Most of the world's youth and war-like violence, however, is concentrated in developing countries, yet there is limited mental health data within affected countries. This study focused on a random community-based sample of 552 impoverished youth ages 6-18 within an informal settlement in Nairobi, Kenya, which experienced war-like violence for a month following the contested presidential election of 2007. Six months after the violence ended, 99 (18%) had PTSD according to the UCLA PTSD Reaction Index (Steinberg, Brymer, Decker, & Pynoos, 2004), and an additional 18 (3%) were found to have partial PTSD due to high overall scores. Kenyan psychologists conducted diagnostic interviews and found the positive predictive value of the assessment tool to be 72% in this sample; the confirmed prevalence was 12%. Similar to other studies worldwide, Criterion C (avoidance) was the limiting factor for diagnosing PTSD according to the DSM-IV-TR, and parent-child agreement was at best fair. The number of traumatic experiences was strongly associated with PTSD outcomes. Differences due to age or sex were not found. The findings indicate the need for universal mental health services for trauma-exposed youth and their families in the impoverished informal settlements of Nairobi, Kenya.