In 35 patients with simple multinodular goiter and with normal preoperative TSH reserve, the mean basal TSH and the mean TSH response to TRH increased significantly 5-15 days and 1-15 months following partial thyroidectomy. The postoperative mean TSH values were also greater than the age-related normal mean values. The incidence of postoperative supranormal TSH values and the incidence of postoperative increase in basal and TRH-stimulated TSH did not differ in the early and late postoperative course. In the latter the FTI had decreased significantly in the euthyroid range as compared to the preoperative value, whereas the mean serum T3 had not increased significantly. The patients were divided into two subgroups according to the anatomo-pathological finding and ensuing type of goiter resection (either selective ablation of non-functioning parenchyma with normal remnants, or resection of functioning and non-functioning parenchyma in the presence of goitrous remnants); the postoperative increase in the mean basal and TRH-stimulated serum TSH values was significant only in the non-selective ablation subgroup. Preclinical hypothyroidism was noted post surgery in 6 patients with supranormal preoperative TSH reserve. The results demonstrate a decreased thyroid reserve, as reflected in a persistent increase in thyrotropic function following resection of simple multinodular goiter in patients from a mildly endemic area. Long-term follow-up and thyroid hormone substitution may chiefly be indicated following non-selective goiter resection, in order to eliminate or prevent preclinical hypothyroidism and to prevent development of recurrent goiter.