Diagnosis of defective pyruvate metabolism can present difficulties in clinical practice. In search of a diagnostic procedure that can give a clear indication of a disturbance of pyruvate metabolism, we have developed an intravenous pyruvate loading test. The loading test was applied to 9 patients with Leigh syndrome. Results and characteristics are described. The test proved to be a sensitive procedure to detect disturbances in pyruvate oxidation. The intravenous pyruvate loading test can be a useful tool in the diagnosis of mitochondrial (encephalo) myopathies.