This paper describes the nature and comorbidity of substance use and other psychiatric disorders in 157 impaired practitioners notified to the Medical Practitioners Board of Victoria from 1983 to 1997. Diagnoses were reduced to three diagnostic categories, viz, substance use disorder (n = 62), other psychiatric disorder (n = 62), and combined substance use and other psychiatric disorder (n = 33). Sixty nine percent of those with other psychiatric disorders suffered either bipolar disorder (n = 27), schizophrenia (n = 22), or depression (n = 17). Psychiatric comorbidity in drug-related diagnoses was 26% and in alcohol-related diagnoses 64%. Sixty five percent of substance use disorders were notified more than 12 months after onset of impairment, and 49% of all practitioners (71% with substance use disorder) did not receive treatment prior to notification. This combined with a high relapse rate (41%) makes some impaired practitioners a risk to their patients.