Most previous studies of attention deficit in ADHD and schizophrenia have used overall measures of inattention that may disguise differences in underlying mechanisms. The present study investigated types of inattentive errors and applied a process view of attention in analyses of Conners' Continuous Performance Test protocols from subjects with ADHD-combined (51), inattentive type (19) and schizophrenia (26). Subjects with ADHD-I had more omission errors and became more inattentive as a function of time on task. Subjects with ADHD-C made more errors of commissions as time passed. In contrast, the performance of the subjects with schizophrenia improved, indicating a training effect. There were no differences in overall attentiveness between the groups. Although all groups were impaired on an overall level, they displayed three distinct patterns of inattention. The ADHD-I group displayed a lethargic inattention characterized by high fatigue. The ADHD-C group showed a hyperactive-impulsive pattern, while the schizophrenia group showed an inability to initially focus attention.