Three infants are described who had nemaline rods on muscle biopsy and isolated deficiency of complex I of the respiratory chain on biochemical analysis. They all manifested failure to thrive from birth, and hypotonia and muscle weakness within the first three months of life. Different genetic defects leading to isolated complex I deficiency have been described associated with a variety of morphological changes on muscle biopsy, but rods have not been described. Nemaline rods have been secondary phenomena in a number of conditions, as well as being the primary abnormality in nemaline myopathy. However, the combination of nemaline rods and complex I deficiency is an association not previously reported.