Samples of adults representative of Tezuitlán, Puebla and Villahermosa, Tobasco (combined N = 561), were interviewed 6, 12, 18, and 24 months after the devastating 1999 floods and mudslides in Mexico. Current DSM-IV PTSD and major depressive disorder (MDD) were assessed with the Composite International Diagnostic Interview. At Wave 1, PTSD was highly prevalent (24% combined), especially in Tezuitlán (46%), which had experienced mass casualties and displacement. Both linear and quadratic effects of time emerged, as PTSD symptoms initially declined but subsequently stabilized. Differences between cities lessened as time passed. Comorbidity between PTSD and MDD was substantial. The findings demonstrate that the international health community needs to be prepared for epidemics of PTSD when disasters strike developing areas of the world.