The Eysenck Personality Inventory and Profile of Mood States were administered to 107 men and 43 women before starting methadone maintenance and again after four months of treatment. Both men and women reported decreases in levels of neuroticism, anxiety, and fatigue after four months of treatment. The women described themselves as less depressed, whereas the men indicated that they felt more vigorous. However, the post-test levels of affect for both sexes were still comparable to those reported for outpatient psychoneurotics. The relationships of certain background characteristics and treatment experiences associated with both sexes' changes in affect were also described.