Lactic acidosis (LA), a rare but life-threatening adverse effect associated with antiretroviral therapy, has been reported with an increasing frequency since the mid-1990s. From June 1994 to June 2002, a total of six patients, four males and two females with a median age of 43 years (range, 30 to 74 years), had been diagnosed with LA. The estimated incidence of LA was 5.1 per 1000 patient-years (PYs) on highly active antiretroviral therapy (HAART) (95% confidence interval [95% CI], 4.5-5.5 per 1000 PYs) and 4.4 per 1000 PY on nucleoside analogues (NAs) (95% CI, 3.9-4.7 per 1000 PYs). Their median body mass index at diagnosis of LA was 17.6 kg/m(2) (range 16.3 to 22.6 kg/m(2)). The median CD4+ lymphocyte count at the initial diagnosis of HIV infection and at the onset of LA was 38 cells/ micro L (range, 4 to 103 cells/ micro L) and 108 cells/ micro L (range, 79 to 224 cells/ micro L), respectively. The most common symptoms were nausea, vomiting, and dyspnoea. All of the patients had findings suggestive of NA-related mitochondrial toxicity, such as myositis, pancreatitis, fatty hepatitis, peripheral neuropathy or lipodystrophy. The prescribed NA related to LA were stavudine in six patients, lamivudine, five, and didanosine, one. Despite treatment, all patients died of persistent circulatory collapse following LA. The median duration from diagnosis to death was eight days (range, 4-17 days). Our report highlights that clinicians caring for patients with AIDS should be alerted to the potentially fatal LA associated with antiretroviral therapy when patients present with low body mass index, lipodystrophy, unexplained abdominal symptoms, dyspnoea, or elevated aminotransferases.