A survey of laboratory investigations in the management of 1007 non-geriatric admissions to a district general hospital adult psychiatric unit showed a wide variation in the number of tests performed, and in the abnormal proportion between test types. Despite a policy of screening on admission, 40% of admissions had no tests. Physical illness attributable to mental illness was common, but rarely serious and usually apparent clinically. Mental illness attributable to physical illness detected by laboratory tests was rare: five cases of thyroid disease in women with affective disorder, and two cases of chest infection with raised white cell count in sustained mania. The findings show that tests are frequently used in circumstances where the result is of no apparent value. Test use might be improved by restricting screening to thyroid function and laboratory urine analysis, each in women. There are particular indications for white cell count in sustained or unexplained relapse of mental illness, and for syphilis serology when disinhibition may have exposed the patient to infection.