Agranulocytosis, although extremely infrequent, is a serious complication of antithyroidal drug therapy in patients with hyperthyroidism. Presently, there is no specific therapy for this life-threatening complication, and recovery time is highly variable. Recently, recombinant human granulocyte colony-stimulating factor (rhG-CSF) was reported to be effective in shortening the recovery time of the neutropenia in patients undergoing chemotherapy. The present study was undertaken to determine the efficacy of rhG-CSF administration in patients with methimazole-induced (MMI) agranulocytosis. Thirty-four patients (7 males and 27 females, ages 16-68 yr) with MMI agranulocytosis were divided into 3 groups: group A (n = 11) was treated with antibiotics only; group B (n = 11) received antibiotics and dexamethasone, 8 mg/day; and group C (n = 12) was treated with antibiotics and im injections of rhG-CSF, 75 micrograms/day. Patients in groups A and B were studied retrospectively. When rhG-CSF became available, patients in group C were studied prospectively. Bone marrow sternal punctures were performed in all group C patients who were then divided into 2 subgroups according to the granulocyte to erythrocyte count ratio (G:E). Group C1 (n = 6) had a G:E ratio of less than 0.5, and group C2 (n = 6) had a ratio of more than or equal to 0.5. Recovery time in all groups was defined as the number of days required for the peripheral granulocyte count to be greater than 1.0 x 10(9)/L. There was no significant difference in recovery time between groups A and B: 10.1 +/- 2.2 and 12.3 +/- 1.9 days (mean +/- SE), respectively. P was not significant; the administration of dexamethasone proved to be ineffective in shortening the time for recovery from peripheral granulocytes. On the other hand, recovery time was significantly shorter in group C (6.8 +/- 1.2 days mean +/- SE) compared with groups A and B (P < 0.05). Group C2 recovered in 2.2 +/- 0.6 days whereas group C1 took much longer, 9.8 +/- 1.3 days (P < 0.001). There was a direct correlation between the G:E ratio and the peripheral leucocyte count, r = 0.806, P < 0.01. Furthermore, rhG-CSF significantly shortened recovery time when the peripheral granulocyte count was greater than 0.1 x 10(9)/L (group C2) compared with patients whose counts were less than 0.1 x 10(9)/L (group C1), 2.2 +/- 0.4 vs. 8.6 +/- 1.3 days, respectively (P < 0.001). These data indicate that administration of steroids is ineffective in shortening the duration of recovery in patients with MMI agranulocytosis.(ABSTRACT TRUNCATED AT 400 WORDS)