Charts of 125 adults (18 to 82 years), referred to an autism expert team for Autism Spectrum Disorder (ASD) assessment, were reviewed to explore the pathway to an adulthood ASD diagnosis. The participants first contacted the mental health care clinic at a median age of 19 years (range 2 to 78 years). Men contacted the clinic slightly earlier than women. The main referral reasons were social problems, feelings of anxiety and mood disturbances. The most common earlier diagnoses were anxiety and mood disorders or psychosis-related disorders. These diagnoses were more common in women than in men. Surprisingly few differences emerged between those who finally received an ASD diagnosis and those who did not. However, those with an ASD diagnosis contacted the clinic a mean of 15 years earlier and less frequently received different former diagnoses, although the type of diagnoses did not differ. The diagnostic criteria that were prevalent during early childhood of these adults did not influence their diagnostic history. A quarter of these clients were known with social problems within the mental health care system, but ASD was not assessed. Hence, the current study shows that the pathways to an adulthood ASD diagnosis are very heterogeneous. (Contains 1 table.)