Despite the importance of clinical judgement in autism spectrum disorder (ASD) assessment, little is currently known about challenges faced by diagnosticians when the client is female, any sex/gender biases during the assessment process, and how these issues affect diagnostic outcomes. Forty-seven ASD diagnosticians completed a questionnaire containing two hypothetical case studies (a ‘male’ and ‘female’ ASD presentation), with sex/gender randomly assigned within each. Diagnosticians reported greater ASD symptom severity when female sex/gender pseudonyms were allocated to the case studies, but their confidence in ASD diagnosis was similar regardless of condition. Diagnosticians identified a large number of challenges associated with assessing females for ASD. Many of these related to sex/gender differences in ASD presentation and limitations of diagnostic instruments.