Severe lactic acidosis has been increasingly reported as a potentially fatal complication of HIV treatment. We report on an asymptomatic HIV-infected woman treated with stavudine, lamivudine and indinavir for one year. She was hospitalized because of progressive dispnoea, oedema, cyanosis and severe lactic acidosis. Arterial blood pH was 6.98, bicarbonate 4.4 mmol/l (normal value 22-26), blood lactate: 29.7 mmol/l (normal value <2.2). Hepatic function was normal. She had an impressively rapid response (within a few hours) to empirical treatment with thiamine (100 mg i.v.). No evidence of sepsis or malabsorption were identified and vitamin B1 level was not tested before thiamine infusion. Three months later she was re-started successfully on nelfinavir plus nevirapine. The rapid response to thiamine infusion deserves a careful attention and such an approach should be considered in similar cases as a support treatment of this potentially life-threatening complication of HIV therapy.