The efficacy and safety of ubiquinone (Q10) and nicotinamide were evaluated in a 6-month open-label trial in patients with the 3243A-->G mitochondrial DNA mutation. Blood lactate and pyruvate concentrations decreased, but there was little clinical improvement. Q10 and nicotinamide were well tolerated, but two patients died suddenly and unexpectedly during the trial. These deaths may have been unrelated to treatment. The unpredictable course of the disease makes evaluation of the clinical response difficult.