A patient with procainamide lupus erythematosus had a large pericardial effusion. As in other reported cases histology revealed a fibrinous mononuclear pericarditis and the pericardial fluid was a serosanguinous inflammatory exudate with a high LDH level and normal glucose concentration. The ANF and LE cell preparation were positive in the fluid but the C3 complement was normal. The frequency of pericarditis is similar in systemic and drug-induced lupus erythematosus yet low complement levels need not occur. Complement activation may therefore be unnecessary for the development of either type of lupus pericarditis.