Although there has been extensive research in the area of Attention-deficit Hyperactivity Disorder (ADHD), the estimated prevalence of this disorder ranges from 1% to 20%. This variability is due in part to variations in how one defines ADHD and the difficulty of precisely measuring the features of the syndrome. Additionally, problems of differential diagnosis and comorbidity of ADHD with other disorders may impact resultant prevalence rates. The present study was conducted in order to examine the impact of various qualitative and quantitative differences in the diagnostic process on the prevalence of ADHD in samples of children in special education and children referred to a tertiary-care specialty clinic for learning disabilities (LD) and ADHD. Of particular interest was the prevalence of ADHD with no other psychopathology (pure ADHD) in the two samples. Children with pure ADHD made up only 15.8% of the special education sample (teacher rating only). In contrast, the prevalence of pure ADHD in the specialty clinic sample ranged from 35.6% (teacher rating only) to 11.9% (parent rating only). Only 5.9% were rated by both teacher and parent as pure ADHD. The clinical and methodological implications of these findings are discussed in terms of the criteria for diagnosis of ADHD and implications for future research.