Peripheral large granular lymphocytes (LGL) were enumerated in normal subjects and patients with autoimmune thyroid diseases. In normal subjects, the percentage of LGL was significantly lower in women (mean +/- s.d., 17.0 +/- 3.6%; n = 35; P less than 0.05) than in men (19.5 +/- 5.3%; n = 20). In untreated patients with thyrotoxic Graves' disease (GD), both the percentage (11.5 +/- 2.7%; n = 12; P less than 0.001) and the absolute count (244 +/- 102/mm3; P less than 0.01) of LGL were significantly lower than those in normal women (17.0 +/- 3.6% and 334 +/- 122/mm3; n = 35). No significant differences from normal controls were observed in the percentages or absolute counts of LGL in patients with euthyroid GD under treatment or in euthyroid or hypothyroid patients with Hashimoto's disease (HD). The percentage of LGL was inversely correlated with the serum levels of T4 and T3 and the free T4 index, but not with the titre of anti-thyroid antibodies, the size of goitre or the degree of proptosis in the group of untreated patients with GD and HD. The decreased levels of LGL in thyrotoxic patients with GD may be related to the self-perpetuation of thyrotoxicosis in patients with this disease.