Objective: Attention deficit hyperactivity disorder (ADHD) is a highly heritable but clinically heterogeneous syndrome. The study examined the familiality and heritability of ADHD subtypes as defined by DSM-IV and by latent-class analysis in a population sample of adolescent female twins. Method: To determine which elements of ADHD cluster together, latent-class analysis was applied to data obtained from parents on the 18 DSM-IV ADHD symptoms in 4,036 female twins age 13-23 years in a population sample identified from the registry of all births in Missouri for the years 1968-1996. Relative risk and odds ratios were used to assess within-subtype and between-subtype familiality and heritability of both DSM-IV and latent-class ADHD subtypes. Results: Latent-class analysis was most compatible with the existence of three mild and three severe classes of ADHD symptoms in the general population. The three severe classes showed moderate overlap with DSM-IV ADHD subtypes. The primarily inattentive and combined subtypes of DSM-IV ADHD co-clustered within families. The primarily hyperactive/impulsive DSM-IV subtype and the individual latent-class analysis subtypes did not co-cluster. Subtypes defined by both approaches were highly heritable. Conclusions: Unlike DSM-IV subtypes of ADHD, latent-class ADHD subtypes appear to be independently transmitted in families. These classes may be more appropriate targets for molecular genetic studies of ADHD.