We report the case histories of two 6-month-old girls, both with young, nonconsanguineous parents, referred to us for suspected blindness. In both cases, Leber's congenital amaurosis was diagnosed. Due to persistently high lactic acid levels in blood, muscle biopsies were taken. Analysis of biopsies revealed that both patients had low levels of complex IV of the mitochondrial respiratory chain; one patient additionally had low levels of complex III. Microscopic and ultrastructural alterations of muscle, typically observed in mitochondrial disorders, were observed only in the second patient. These observations raise the possibility that at least some cases of Leber's congenital amaurosis may be due to alterations in the mitochondrial respiratory chain.