The origin of the psychiatric illnesses observed in drug abusers is often unclear. This study examines the causal relation between drug abuse and specific psychiatric disorders. Fifty-one male veterans first seen in 1972, who were admitted at least once per year for six consecutive years for inpatient drug-abuse treatment, underwent psychiatric assessments at each admission. Eleven men mainly used stimulants, 14 depressants, and 26 opiates. Initial psychiatric examinations showed low symptom levels in all groups but no statistically significant differences among them. By the end of six years, five of the stimulant users had psychoses, and eight of the depressant users had serious depression. The narcotics users showed no change in psychopathology. Differences between the groups were significant at the 0.01 level. These changes were not due to acute toxic reactions, but our data suggest that abuse of particular drugs has a major role in the development of specific psychiatric illnesses. The possibility that different preexisting personality disorders lead to different kinds of drug abuse cannot be excluded.