The current DSM-IV defines ADHD as a disorder with two separate underlying symptom dimensions and allows for the diagnosis of three subtypes: primarily hyperactive-impulsive, primarily inattentiveness, and combined. This study found that persistent genetic influences were important in the development of ADHD subtypes from childhood to adolescence. One common genetic component was found, i.e., persistent cross-subtype influences, that affected hyperactivity-impulsivity and inattentiveness over time. This could be interpreted as persistent genetic influences on ADHD of the combined type. The finding that combinations of ADHD subtype behaviors persist with age, along with specific subtypes such as hyperactive-impulsive and inattentiveness, suggest a genetic basis for the DSM-IV classification of ADHD subtypes. The study sample included all 1,480 twin pairs born in Sweden between May 1985 and December 1986. Parents responded to mailed questionnaires when the twins were 8 to 9 years old, 13 to 14 years old, and 16 to 17 years old. ADHD symptoms were measured with dimensional scales of hyperactivity-impulsivity and inattention derived from a checklist of items based on the DSM symptoms of ADHD. 4 tables, 1 figure, and 42 references