Clinical samples of attention deficit hyperactivity disorder (ADHD) have been dominated by males. Consequently, female manifestations and sex differences have been relatively neglected in the extensive ADHD research. Because ADHD is so common (3% to 5% of school children) and chronic (lifelong in many cases), even a small proportion of females multiplied by such a large base means hundreds of thousands of girls and women with ADHD, a significant public health problem. An NIMH conference concluded that research is needed not only on sex differences related to ADHD, but also on manifestations of ADHD in females as such. Areas of focus should include differences in life course (sex-differential age effects); effects of hormones; effects of ADHD parenting (in utero and postnatal) on the next generation; response to and implications for design of psychosocial treatment; effects of differential comorbidity; normative "background" sex differences that influence the manifestation of ADHD; differences in development of verbal fluency and social behavior; possible interactions of sex and ethnicity; a prospective study of both sex offspring of ADHD adults; and such methodological issues as appropriate instruments and diagnostic thresholds, power to prevent false negatives, valid impairment measures, validity and reliability of child self-reports, and more inclusive samples (all three subtypes: inattentive, hyperactive-impulsive, and combined).