Objective: The goal of our study is to assess the frequency, risk factors, and features of hyperlactatemia in a cohort of about 1,000 patients with HIV disease. Methods and Results: In a prospective 6-month case-control study involving about 1,000 HIV-infected patients, a 35.9% crude frequency of serum lactic acid abnormalities was found among the 743 evaluable individuals. However, no difference emerged between patients with hyperlactatemia and control subjects as to multiple investigated factors, except a longer duration of antiretroviral therapy and highly active antiretroviral therapy, a lipodystrophy syndrome, and elevated serum triglyceride, creatinphosphokinase, and aldolase levels, which involved more significantly patients with altered lactatemia compared with those who had not serum lactic acid anomalies. The 52 patients who developed repeated hyperlactatemia and the 5 individuals who suffered from a very high (grade 4 toxicity) rise of serum lactic acid levels, did not show different supporting factors and course, compared with all the remaining subjects with isolated or low-level hyperlactatemia. No significant relationship was found with the administration and length of use of each single anti-HIV nucleoside analogue. Conclusion: Hyperlactatemia, although frequently transient and asymptomatic, is still an underestimated problem of HIV infection and antiretroviral therapy, which deserves careful attention on the epidemiologic, etiopathogenetic, and clinical point of view, to plan therapeutic and preventive strategies of this possibly life-threatening complication of treated HIV disease.