Background: A key issue in the nosology of attention deficit hyperactivity disorder (ADHD) has concerned whether the DSM-IV combined (ADHD-C) and primarily inattentive (ADHD-PI) subtypes are in fact distinct disorders, or instead are fairly closely related, perhaps differing only in severity. Pertinent to this question, but in short supply, are data on family psychiatric history. Method: We present new data followed by a meta-analysis. For the new data 210 children were well-characterized via structured interview and multi-informant ratings as ADHD-C or ADHD-PI. Three hundred and thirty-five parents completed structured diagnostic interviews to determine whether they had ADHD-C or ADHD-PI. Comorbid disorders were also assessed. For the meta-analysis, 6 existing family-history data sets using DSM-IV subtypes were pooled and analyzed (total N = 4120). Results: The new family study found support for subtype-specific transmission, but the effect was small. Girls with ADHD-C had more severe family psychiatric histories (non-ADHD disorders in parents) than girls with ADHD-I, but this subtype effect did not occur for boys. The meta-analysis of 6 data sets implemented a model-fitting methodology. Despite inconsistent results across individual studies, the pooled data consistently suggested that (1) some degree of subtype specific transmission is occurring, (2) the effect is small, (3) non-specific (gradient) transmission also occurs, specifically in families of children with ADHD-C. Conclusion: The much-debated relation between these two most prevalent ADHD subtypes is best modeled via two processes transmitting in families. One process is distinct for the two syndromes, and the other is shared. We briefly suggest neuropsychological models that would account for this state of affairs and suggest future research to refine the nosology with these findings in mind.