This report presents national estimates of the prevalence of diagnosed attention deficit disorder (ADD) and/or learning disability (LD) in children in the United States. Differences in the prevalence of these conditions are examined for children with selected sociodemographic characteristics. Estimates in the report are based on data from the 1997 and 1998 National Health Interview Survey that included 8,647 children (ages 6-11). Results indicate that in 1997-98 more than 2.6 million children were reported to have had a diagnosis of ADD or LD. A diagnosis of only ADD was reported for 3% of children, a diagnosis of only LD for 4%, and a diagnosis of both conditions for 4%. The prevalence of ADD with or without LD was greater for boys. Having health insurance was associated with only ADD. Living in a low-income or mother-only family occurred more often among children with a diagnosis of LD. Children with LD were nearly five times more likely to be in special education than children with a diagnosis of only ADD. Children with ADD more often had contact with a mental health professional, used prescription medication regularly, and had frequent health care visits. Appended are: Technical Notes; and Definition of Terms. (Contains 32 references and 4 text tables.) (Author/CR)