An epidemiological study on the effects of type-A behavior on the relationship between paced work and mood disturbance was conducted. The cohort consisted of 6591 postal workers (SIC-4311) engaged in machine paced mail sorting operations, and 5722 postal workers engaged in non machine paced jobs. Questionnaires were sent to all subjects and data on patterns of behavior was obtained through an activity scale. All subjects were interviewed and psychological disturbance was measured in terms of tension/anxiety, depression, anger/hostility, fatigue/inertia, confusion, and activity. Stress not associated with the job was measured with a social readjustment rating scale. Results were analyzed for statistical significance by factorial analyses. Of the six different mood measures analyzed, no instance was evident in which type-A and type-B workers of the same sex differed significantly in magnitude of disturbance reported. Data revealed that type-A disposition had a direct effect on mood; type-A workers reported more disturbance that their type-B counterparts. Type-A male workers (but not type-A females) demonstrated personality traits that predisposed them to selection into stressful jobs. The author concludes that the relationship between machine pacing and type-A behavior is an important criterion in measuring behavior predisposition.