A study on the course of hyperlactataemia during highly active antiretroviral therapy (HAART) and the association between hyperlactataemia and antiretroviral drugs was conducted at the outpatient department, Rigshopitalet, Copenhagen. Lactate levels were monitored in 848 patients during a study period of 1 y. Longitudinal analysis was performed on all human immunodeficiency virus-1-infected patients who had plasma lactate > 2.1 mM. Hyperlactataemia was found in 178 patients (21%), of whom 7 patients needed treatment modification, owing to symptomatic hyperlactataemia in 3 and neuropathy in 4 patients, while 171 remained on unchanged therapy. Lactate levels increased in 20 patients during the study period, but the increases were modest with a mean of 0.6 mM (range 0.1-1.7 mM). The association between antiretroviral drugs and hyperlactataemia was studied using logistic regression in 263 patients with data on their treatment regimen available in electronic form. Only stavudine and ritonavir were significantly associated with hyperlactataemia, with odds ratios of 5.1 and 2.6, respectively. In conclusion, symptomatic hyperlactataemia is uncommon, while asymptomatic hyperlactataemia is a frequent and apparently benign condition unlikely to progress to lactic acidosis. A significant association between stavudine and hyperlactataemia was confirmed. The unexpected association between ritonavir and hyperlactataemia will need confirmation in future studies.