Background Studies of survivors of the September 11, 2001 attacks on the World Trade Center in New York City suggest that postdisaster depressive disorders may be at least as prevalent, or even more prevalent, than posttraumatic stress disorder (PTSD), unlike findings from most other disaster studies. The relative prevalence and incidence of major depressive disorder (MDD) and PTSD were examined after the 9/11 attacks relative to trauma exposures. Methods This study used full diagnostic assessment methods and careful categorization of exposure groups based on DSM-IV-TR criteria for PTSD to examine 373 employees of 9/11-affected New York City workplaces. Results Postdisaster new MDD episode (26%) in the entire sample was significantly more prevalent (p <.001) than 9/11-related PTSD (14%). Limiting the comparison to participants with 9/11 trauma exposures, the prevalence of postdisaster new MDD episode and 9/11-related PTSD did not differ (p =.446). The only 9/11 trauma exposure group with a significant difference in relative prevalence of MDD and PTSD were those with a 9/11 trauma-exposed close associate, for whom postdisaster new MDD episode (45%) was more prevalent (p =.046) than 9/11-related PTSD (31%). Conclusions Because of the conditional definition of PTSD requiring trauma exposure that is not part of MDD criteria, prevalence comparisons of these two disorders must be limited to groups with qualifying trauma exposures to be meaningful. Findings from this study suggest distinct mechanisms underlying these two disorders that differentially relate to direct exposure to trauma vs. the magnitude of the disaster and personal connectedness to disaster and community-wide effects.