We have evaluated the effects of treatment with riboflavin in five patients with a mitochondrial myopathy, associated with a complex I (NADH dehydrogenase) deficiency. Two patients suffered from a clinically pure myopathy and the other patients presented with encephalomyopathic features. Treatment with riboflavin resulted in a clear clinical improvement in the two patients with the myopathic form of complex I deficiency. However, only one of the patients with the encephalomyopathic form improved during therapy. In three of the four patients in whom complex I activity in muscle tissue has been determined again during therapy, complex I activity appeared to be normalized. The clinical effects of treatment in this group of patients do not correlate well with normalization of complex I activity.