Thyroid function screening may be of value in psychiatric patients. The present study was undertaken to determine the incidence of thyroid abnormalities in psychiatric patients during short-term admissions. Results of thyroid function tests disclosed an elevated free thyroxine index (FT4I) and free triiodothyronine index (FT3I) in 19% of 386 psychiatric patients during short-term admissions to Minneapolis Veterans Administration Medical Center. Elevated test results were particularly common in paranoid schizophrenics (38%) and in patients with amphetamine abuse (32%). On retesting, at two to three weeks after hospitalization, the FT4I and FT3I had returned to normal in 13 of 16 and 17 of 17 patients, respectively. Undiagnosed hypothyroidism was found in four patients with a diagnosis of manic-depressive psychosis on hospital admission. Serial sampling needs to be undertaken to assess the most appropriate time for thyroid function testing after short-term psychiatric admission.