Objective: Unresolved questions exist concerning diagnosis of ADHD. First, some studies suggest a potential overdiagnosis. Second, compared with the male-female ratio in the general population (3:1), many more boys receive ADHD treatment compared with girls (6-9:1). We hypothesized that this occurs because therapists do not adhere to "Diagnostic and Statistical Manual of Mental Disorders" (4th ed.; "DSM-IV") and International Classification of Diseases (10th rev.; ICD-10) criteria. Instead, we hypothesized that, in accordance with the representativeness heuristic, therapists might diagnose attention-deficit/hyperactivity disorder (ADHD) if a patient resembles their concept of a prototypical ADHD child, leading therapists to overlook certain exclusion criteria. This may result in overdiagnosis. Furthermore, as ADHD is more frequent in males, a boy might be seen as a more prototypical ADHD child and might therefore receive an ADHD diagnosis more readily than a girl would. Method: We sent a case vignette to 1,000 child psychologists, psychiatrists, and social workers and asked them to give a diagnosis. Four versions of the vignette existed: Vignette 1 (ADHD) fulfilled all "DSM-IV"/ICD-10 criteria of ADHD. Vignettes 2-4 (non-ADHD) included several ADHD symptoms but stated other ADHD criteria were nonfulfilled. Therefore, an ADHD diagnosis could not be given. Furthermore, boy and girl versions of each vignette were created. Results: In Vignettes 2-4 (non-ADHD), 16.7% of therapists diagnosed ADHD. In the boy version of these vignettes, therapists diagnosed ADHD around 2 times more than they did with the girl vignettes. Conclusions: Therapists do not adhere strictly to diagnostic manuals. Our study suggests that overdiagnosis of ADHD occurs in clinical routine and that the patient's gender influences diagnosis considerably. Thorough diagnostic training might help therapists to avoid these biases. (Contains 3 tables and 1 figure.)