Long-term treatment with pivampicillin and pivmecillinam for 6-24 months in five adults and one child reduced the total serum carnitine concentrations to 3.7-14.0 mumol/l (reference value 25-66 mumol/l). The muscle carnitine was reduced to 0.3-0.7 mumol/g wet weight (reference value 3-5 mumol/g) in two cases. All patients had asthenia and muscle symptoms with weakness and pain. One showed signs of carnitine depletion in the liver, with increased secretion of dicarboxylic acids (C6, C8, C10) in urine and limited ketone body formation during prolonged fasting (32 hours). The serum carnitine increased slowly after cessation of therapy and reached normal concentrations after 6-12 months. All symptoms caused by carnitine depletion disappeared after the serum carnitine reached 20 mumol/l. This was achieved on a normal diet without carnitine supplement.