Agranulocytosis is a rare complication of antithyroid drugs, and the aetiologies of community-acquired, life-threatening infections in patients taking these drugs have not previously been systematically described. Of 5653 hyperthyroid patients treated with antithyroid drugs at National Taiwan University Hospital between January 1987 and December 1997, 13 (0.23%) developed agranulocytosis with life-threatening infections. The most common presentations were fever (92%) and sore throat (85%). Initial clinical diagnoses were acute pharyngitis (46%), acute tonsillitis (38%), pneumonia (15%) and urinary tract infection (8%). Positive blood cultures from six patients yielded Pseudomonas aeruginosa (3), Escherichia coli (1), Staphylococcus aureus (1), Capnocytophaga species (1). Two patients died of uncontrolled infection, thyroid storm and multiple organ failure. Cases of antithyroid-drug-induced agranulocytosis in the English language literature are reviewed; Gram-negative bacilli, including Klebsiella pneumoniae (4 patients) and P. aeruginosa (3), were the most common pathogens in clinical isolates. Our observation and review suggest that broad-spectrum antibiotics with anti-pseudomonal activity should be given to patients with antithyroid drug-induced agranulocytosis who present with severe infection.