This study consists of 2 steps: Step 1 (screening) and Step 2 (treatment). Patients are screened during Step 1 for sustained hyperlactatemia and Step 2 eligibility. Patients with sustained hyperlactatemia but no symptoms that indicate high risk of progression to lactic acidosis and with no plans to change existing NRTI-containing antiretroviral therapy may enter Step 2. A fasting, nonexercise, venous lactate level is obtained at Step 2 entry for use as a baseline measurement, and every patient receives high-dose oral vitamin B1 and B2 therapy for 4 weeks. Fasting, nonexercise, venous lactate levels are measured at Weeks 1, 2, and 4 to observe the kinetics of changes in lactate levels on study treatment.