Drug-induced agranulocytosis (DIA) is often caused by antithyroid drugs. We retrospectively studied the use of granulocyte colony-stimulating factor (G-CSF) therapy in antithyroid-DIA. Data for 20 patients (10 treated with G-CSF) with antithyroid-DIA (neutrophil count <0.5x10(9)/l) were extracted from a cohort study of DIA patients (n=110). G-CSF (300 microg/day subcutaneously) was used where the neutrophil count was <0.1x10(9)/l, or the patient was aged >70 years, or there were severe features of infection or underlying disease. Mean patient age was 62 years (range 34-87); sex ratio (M/F) was 0.05. Carbimazole (n=19) and benzylthiouracile (n=1) were the causative drugs, at mean doses of 30 mg/day (range 20-60) and 100 mg/day (range 50-150), respectively, for a mean of 37 days (range 31-90). Antithyroid drugs were prescribed for Graves' disease (n=8), thyrotoxicosis related to amiodarone intake (n=6) and multinodular goitre (n=6). Clinical features included isolated fever (n=7), pneumonia (n=5), septicaemia or septic shock (n=5) and acute tonsillitis (n=3). Mean neutrophil count was 0.07+/-0.1x10(9)/l. No patient died. Mean durations of haematological recovery, antibiotic therapy and hospitalization were significantly reduced with G-CSF: 6.8+/-4 days vs. 11.6+/-5; 7.5+/-3.8 days vs. 12+/-4.5; and 7.3+/-4.8 days vs. 13+/-6.1, respectively (all p<0.05). G-CSF induced flu-like symptoms in 30% of patients, but reduced overall costs.