Substance use is prevalent in patients with psychiatric disorders and may cause severe symptoms in addition to complicating the diagnosis of psychiatric disorders. The aims of the study were to find the prevalence in use of alcohol, drugs, benzodiazepines, hypnotics, opiates and stimulants, and to find the prevalence of substance use disorders at admission to an acute psychiatric department receiving all admissions from a catchment area. Patients were interviewed about use of medications and intoxicating substances during the last week before admission in 227 consecutive admissions. Urine samples were analysed with the liquid chromatography with mass spectrometry (LC-MS) method. Use of substances was determined from reported use and findings in urine samples. Diagnoses were set at discharge according to ICD-10 research criteria. In 81.9% of the admissions, the patient had used alcohol, drugs, benzodiazepines, hypnotics, opiates or stimulants prior to admission. More men used alcohol, cannabis and stimulants, whereas more women used benzodiazepines. In 31.7% of the admissions, 49.5% of men and 16.4% of women, the patients had a substance use disorder (ICD-10, F10-19). Patients with substance use disorders had a shorter stay in hospital than other patients, and patients with no psychiatric disorder other than substance use disorders had a median length of stay of 2 days. Most patients had used psychoactive substances before admission to the acute psychiatric department, and half of the men had a substance use disorder.