OBJECTIVE--To study the effect of induced carnitine depletion on myocardial structure and function. SUBJECTS AND DESIGN--7 healthy adult volunteers given 1200 mg pivmecillinam per day for 7-8 weeks were studied by echocardiography before and after 7-8 weeks of treatment and a 15 months follow up after the treatment period. SETTING--Teaching hospital. MAIN OUTCOME MEASURES--Carnitine concentration in serum, urine, and muscle and echocardiographic measurements. RESULTS--After 7-8 weeks of treatment the median free serum carnitine concentration was reduced to 7% and the median total muscle carnitine concentration to 46% of the pretreatment levels. The median diastolic interventricular septum thickness decreased by 14% (mean 26%, P = 0.028) and the median left ventricular mass by 10% (mean 20%, P = 0.018). Fifteen months later these dimensions had increased but not completely returned to pretreatment values. CONCLUSIONS--Extended treatment with pivalic acid containing antibiotics causes carnitine depletion which may lead to changes in cardiac structure.