Numerous disorders are associated with euthyroid sick syndrome (ESS). This retrospective study examines the incidence and circumstances of ESS among 3188 psychiatric inpatients. There were 324 patients (10.2%) who met strictly defined criteria for ESS. Of these, 95 were hyperthyroxinemic (HT), 6 were hypothyroxinemic, 179 had mildly elevated thyroid-stimulating hormone (HTSH), and 47 had suppressed TSH. All were classified by DSM-III-R discharge diagnoses, encompassing five categories. chi 2 tests of significance of the 95 HT and 179 HTSH subjects revealed the following: 1) no relationship with age or gender; 2) the frequencies of HT and HTSH differed significantly (p < .05 and p < .01, respectively) across the five psychiatric categories; 3) HT frequency was highest in mood disorders (HT in mood versus others p < .02); and 4) HTSH frequency was highest in substance abuse (HTSH in substance abuse versus others p < .02). In conclusion, ESS is common in psychiatric inpatients, especially HT and HTSH; pathophysiologic mechanisms may vary according to psychiatric diagnosis.