This cross-sectional study of acute psychiatric admissions compared physicians' assessments of recent substance intake and on-site urine testing with comprehensive laboratory drug analyses. The sample comprised 325 consecutive admissions from 2 acute psychiatric wards. Physicians on call were asked to judge if the patient had recently taken benzodiazepines, opiates, alcohol, amphetamines, cannabis, or cocaine. Blood and urine samples were obtained and analyzed with chromatographic laboratory methods for a wide range of substances. A routine on-site urine screening test was performed in 92 of the cases. Physicians' assessments and on-site urine testing were compared with the reference standard of laboratory analyses. The sensitivity of the physician's assessment was highest for amphetamines (76%), followed by benzodiazepines (61%), opiates (57%), cannabis (55%), and cocaine (50%), whereas specificity was greater than 90% for all substances. The sensitivity of the on-site test ranged from 76% for amphetamine to 97% for cannabis, and specificity ranged from 82% for cannabis to 100% for cocaine. The study indicates clinical underdetection of recent substance intake among acute psychiatric admissions. On-site urine testing identified substance use that was not recognized by the physician's initial assessment, although specificity for cannabis and benzodiazepines was low. Chromatographic methods, which offered important supplementary information about substance use, should be considered for the routine screening of acutely admitted psychiatric patients.