BACKGROUND The psychological aftermath of the Chernobyl accident is regarded as the largest public health problem unleashed by the accident to date. Yet the mental health of the clean-up workers, who faced the greatest radiation exposure and threat to life, has not been systematically evaluated. This study describes the long-term psychological effects of Chernobyl in a sample of clean-up workers in Ukraine. METHODS The cohorts were 295 male clean-up workers sent to Chernobyl between 1986 and 1990 interviewed 18 years after the accident (71% participation rate) and 397 geographically matched controls interviewed as part of the Ukraine World Mental Health (WMS) Survey 16 years after the accident. The World Health Organization (WHO) Composite International Diagnostic Interview (CIDI) was administered. We examined group differences in common psychiatric disorders, suicide ideation and severe headaches, differential effects of disorder on days lost from work, and in the clean-up workers, the relationship of exposure severity to disorder and current trauma and somatic symptoms. Analyses were adjusted for age in 1986 and mental health prior to the accident. RESULTS Relatively more clean-up workers than controls experienced depression (18.0% v. 13.1%) and suicide ideation (9.2% v. 4.1%) after the accident. In the year preceding interview, the rates of depression (14.9% v. 7.1%), post-traumatic stress disorder (PTSD) (4.1% v. 1.0%) and headaches (69.2% v. 12.4%) were elevated. Affected workers lost more work days than affected controls. Exposure level was associated with current somatic and PTSD symptom severity. CONCLUSIONS Long-term mental health consequences of Chernobyl were observed in clean-up workers.