In an attempt to evaluate the diagnostic potential of the urine test for phosphoryltransferase inhibition in subacute necrotizing encephalomyelopathy (Leigh's disease), the test has been performed on 511 individuals. Of these, 137 are known not to have the disease. The rate of false positives in controls has been 6.4 percent. More than a third of the 39 parents of affected children had positive (inhibitory) urines (p < 0.001). All 16 patients with proved Leigh's disease whose urine was tested within two weeks of collection had positive urines (p < 0.0001). These findings support the theory that an inhibitor of thiamine triphosphate formation exists in Leigh's disease, the presence of which is genetically determined. The urine test is of some use clinically; it appears more effective in ruling out the disorder than in establishing the diagnosis.