Objective: We sought to identify clinical discriminators between predominantly mood disordered and predominantly autism spectrum disordered research subjects that may reflect phenotypic state and treatment response characteristics. Method: Participants were 26 boys and 4 girls aged 2 to 18 years (Mean Age = 7.70). Subjects with DSM-IV diagnoses of Major depression (N = 2), Bipolar Disorder (N = 4) and Mood Disorder not otherwise specified (NOS) (N = 11) represented the mood disorder group (MD) (N = 17, Mean Age = 8.2) and those with diagnoses of Autistic Disorder (N = 1), Aspergerбпs Disorder (7) or Pervasive Developmental Disorder (NOS) (N = 3) comprised the autism spectrum disorder (ASD) group (N = 9, Mean Age = 6.8). Primary outcome measurements were continuous actigraphic measurements collected over one to three week periods. Secondary outcomes included personality and observational measurements. Personality characteristics reflected significant cross-group impairments related to self-control and self-discipline and differed relative to intellectual measures. Observational measurements reflected greater general impairments among the ASD group. Results: Predominantly mood disordered children demonstrated greater impairments related to sleep (P = 0.000) and sleep onset latency (P = 0.000) and were more active than ASD children during evening periods (P =