Abstract The objective of this paper is to present data from the Ontario Child Health Study on the prevalence of attention deficit disorder with hyperactivity (ADDH). The overall prevalence of ADDH was 9.0% in hoys and 3.3% in girls. There were no significant differences in the prevalence of ADDH by age or urban-rural status, but the disorder was significantly more common in boys than in twirls. The prevalence of various subtypes ADDH was also explored: attention deficit with and without hyperactivity, situational vs pervasive ADDH, and ADDH with and without other disorders. The clinical implications of these findings are discussed.