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The objective of this study was to compare the prevalence and types of psychiatric disorders in a clinical sample of 118 individuals diagnosed as children with infantile autism (IA) with psychiatric disorders in 336 matched controls from the general population using data from the nationwide Danish Psychiatric Central Register. The average observation time was 32.5 years, and mean age at follow-up was 40.6 years (range 25-55 years). Of the 118 individuals with IA, 57 (48.3%) had been in contact with psychiatric hospitals (inpatient hospitalization or outpatient visits) during the follow-up period, compared with 20/336 (6.0%) in the control group (p < 0.0001). This observation should alert general psychiatrists to the possibility of additional treatable psychiatric disorders occurring in individuals with IA. Of the 118 individuals in the IA group, 20 individuals (17%) had been given a comorbid psychiatric diagnosis during the observation period, compared with 9 individuals (2.7%) in the control group. Of the subjects with IA, 3.4% had received a diagnosis of schizophrenia (F20) at least once since the index admission in childhood, 0.8% had been diagnosed with delusional disorder (F22), 0.8% with acute psychotic disorder (F23), and 1.6% with unspecified non-organic psychosis (F29). In the control group, 0.9% had been diagnosed with schizophrenia (p = 0.08). In the group with IA, 3.4% had received a diagnosis in the broad category of affective disorders compared with 1.2% in the control group (p = 0.21). Issues associated with using registers in the ascertainment of co-occurring psychiatric disorders in IA are discussed.