Until recently, ADHD has been viewed as a developmental disorder largely affecting young boys. Today, many consider the disorder to be overdiagnosed among boys and men and underdiagnosed among girls and women. The primary focus of the current study was to examine clinician gender bias as it relates to the underdiagnosis of the disorder among girls and women. The role of clinician gender, gender competency, and practitioner type on diagnosis was also explored. A within-subjects vignette design was used in which clinicians evaluated boys and girls presenting with inattentive, hyperactive or impulsive, combination, or non-ADHD behaviors. Across the ADHD vignettes, chi-square analyses indicated significantly higher misdiagnosis rates than expected, showing that diagnostic inaccuracy was common across all presentation types. Clinicians were least accurate in diagnosing the inattentive presentation vignette, for both boys and girls, which suggested that difficulty recognizing this subtype, rather than clinician gender bias, played the largest role in diagnostic errors. Girls were underdiagnosed in the inattentive and hyperactive or impulsive conditions, although boys were also underdiagnosed at similar rates. The data did not support the idea that boys were systematically overdiagnosed. Regression analyses showed that clinician gender and self-reported gender competency did not predict diagnostic accuracy, and psychologists and pediatricians performed at similar levels. Overall, the findings suggested that challenges in identifying the inattentive subtype of ADHD had a stronger influence on diagnostic outcomes than clinician gender bias. These results highlight the need for greater attention to inattentive ADHD and raise broader concerns about high rates of misdiagnosis across clinicians.