Eight patients with chronic Q fever endocarditis were treated with tetracycline for up to 40 months. In addition, five of these patients received co-trimoxazole. Six patients had prosthetic valves. Two patients who had Q fever endocarditis on their native valves required valve replacement because of haemodynamic difficulties: in only one did the Q fever endocarditis contribute to the haemodynamic difficulty. One patient died. It is suggested that medical treatment is continued until clinically and haematologically there is no evidence of endocarditis and the Q fever phase 1 antibody titre is less than 200. No recurrence of Q fever endocarditis has been detected in three of our patients who have now stopped treatment.