This study examines the impact of patient and clinician gender and gender roles on a case of undifferentiated gender meeting the minimum diagnostic criteria for APD. The aim was to provide an in-depth and thorough assessment of how patient gender and clinician characteristics may influence a situation such as diagnostic bias in PDs. Psychologists (N = 167) read two cases, including the target case, and provided symptom assessment and diagnosis. Undifferentiated gender was used as a criterion to assess over- or under-diagnosis bias, and diagnoses made based on symptoms were compared to the assigned diagnosis. The gender roles of clinicians were assessed using the Bem Sex Role Inventory-Short Form. The results showed that bias occurred when the patient’s gender (female) did not match the gender weighting of the case (male) symptoms, but the direction of bias was consistent with gender roles (gender-role inconsistent diagnosis – underdiagnosis, gender-role consistent diagnosis – (overdiagnosis). Path patterns of over- and underdiagnosis bias were explored using stereotypic equations. Patient gender had a direct effect on diagnostic assessment and clinician gender had an indirect effect through symptom assessment.