Objective: Previous research has shown that first responders exhibit elevated rates of psychopathology. Factors predicting the development of this psychopathology, however, remain understudied. This study longitudinally examined predictors of posttraumatic stress disorder (PTSD), depression, and anxiety symptoms in first responders. Method: Participants included 135 emergency medical service (EMS) providers. Multiple linear regressions were used to model predictors of change in PTSD, depression, and anxiety symptomatology from baseline to 3-month follow-up. Baseline levels of social support, sleep, emotional stability, and perceived stress were examined as potential predictors. Results: Results revealed that (a) increases in PTSD symptoms, (b) increases in depression symptoms, and (c) increases in anxiety symptoms at 3-month follow-up were each predicted by worse sleep and lower social support at baseline. In particular, the sleep subscale of disturbed sleep and the social support subscale of appraisal appeared to be driving these effects. Conclusion: These results highlight the importance of social support and sleep hygiene in protecting against increases in psychopathology symptoms in EMS providers, and set the stage for future interventions to target sleep disturbances and encourage deeper social connections in order to foster resilience in first responders. (PsycInfo Database Record (c) 2020 APA, all rights reserved).