Objective To evaluate carnitine (3-hydroxy-4-N-trimethyl-ammoniobutanoate) deficiency in AIDS patients by measuring serum total, free and short-chain carnitine concentrations. Design We conducted an open study. Setting All patients were seen at the Infectious Diseases Clinic, Università ‘La Sapienza’, Rome, Italy. Patients, participants Twenty-nine AIDS patients, aged 27–41 years, with a previous history of drug use; and 14 healthy age-and sex-matched controls were studied. Interventions Study subjects were administered 500–800 mg zidovudine daily for 2 to 28 months (8 × 6 months). Main outcome measures Carnitine deficiency was suspected in study participants prior to data collection because of previously reported cardiac symptoms, muscle weakness, hypometabolism and/or cachexia. Results A marked decrease in total and free carnitine was observed in 21 (72%) subjects. Nine of these patients also had low levels of short-chain carnitine. Conclusions AIDS patients may become carnitine-depleted and therefore at risk for alterations in fatty-acid oxidation and energy supply.