Internal consistency of the major diagnostic categories for children was assessed, using the symptom scale scores of the Child Assessment Schedule. Alpha coefficients were calculated for three samples: 116 nonpsychotic psychiatrically disturbed children, 63 children with cystic fibrosis, and 177 children from a community based sample. For the psychiatric sample, a high level of internal consistency was demonstrated for all the symptom scales (i.e., attention deficit, conduct, anxiety, and depression). For the nonpsychiatric samples, the attention deficit and depression scales were reliable, with lower levels of endorsement and more variability observed for the other scales. These results are supportive of the clustering of diagnostic criteria present in DSM-III.