Background: Estimates of psychoactive substance use among acutely admitted psychiatric patients vary among studies, and few have used comprehensive laboratory methods. Aims: This study used chromatography-based analyses of blood and urine to identify the rates of substance use among acute psychiatric admissions, and to study the associations with socio-demographic variables, clinical characteristics and patients' reports of symptoms, substance use and need for treatment. Methods: A cross-sectional study was conducted in 2006/2007 in Oslo, Norway. Blood and urine samples were collected from 298 acute psychiatric admissions and extensively analysed for alcohol, medicinal and illicit drugs. Psychotic symptoms were assessed with the positive subscale of the Positive and Negative Syndrome Scale. Patient self-report questionnaires included the Alcohol and Drug Use Disorder Identification Tests. Patients were also asked if they needed professional help for substance use. Results: Psychoactive substances were detected in 63% of the 298 admissions, medicinal drugs in 46%, alcohol in 12% and illicit drugs in 28%. Patients using alcohol had a high suicidal risk score at admission and the shortest length of stay (median 1 day). Use of illicit drugs was associated with psychotic symptoms and readmission. Self-report questionnaires indicated harmful use of alcohol for half of the patients and of other substances for one-third. A need for professional help for substance use was reported by one-third of patients. Conclusion: Given the high rates of substance use and the important clinical associations, drug screening seems warranted in acute psychiatric settings. Interventions designed for substance-using patients should be developed and integrated.