A 15-month-old girl with propionic acidemia presented a failure to thrive and muscular hypotonia, who had been treated with diets low in leucine, valine, isoleucine, threonine and methionine. Supplementation with leucine gave her a catch up growth and brought about an improvement in muscular hypotonia in parallel with restoration of plasma leucine which had been below normal range during the treatment with the above diets. Oral loading test of leucine produced no ketoacidosis. These findings indicate that the restriction of leucine is not necessary in the dietary management of propionic acidemia.