BACKGROUND Occupational therapists' ability to develop diagnostic-specific ADL intervention strategies is facilitated by an understanding of the ADL abilities of diagnostic groups. OBJECTIVE To determine whether there are significant differences in ADL ability and ADL skill profiles between samples of persons with bipolar disorder depression (n=158), bipolar disorder mania (n=200), and schizophrenia (n=200). Methods. All participants were assessed using the Assessment of Motor and Process Skills (AMPS). Mean ADL ability and skill item calibrations were compared between the three diagnostic samples. RESULTS No clinically significant differences were found in mean ADL ability. The ADL process skill item "Attends" was more difficult for those with bipolar disorder mania than for those with bipolar disorder depression. This difference did not disrupt the measurement model. CONCLUSIONS The findings fail to support the idea that valid predictions of skill performance can be made on the basis of psychiatric diagnosis. Thus, occupational performance must be assessed individually. The findings also provide solid evidence that valid measures of ADL ability are generated when persons with bipolar disorder and schizophrenia complete the AMPS. Intervention strategies are recommended to address limitations in the skill Attends among persons with bipolar disorder manic episode.