Myotonic dystrophy is associated with progressive muscular atrophy. In order to determine the mechanism of muscle wasting in this condition, we measured fractional mixed skeletal muscle protein synthesis in the postabsorptive state in 8 patients with myotonic dystrophy, and compared the results with those of 10 normal subjects. Fractional muscle protein synthesis was determined by measuring the increment of 13C leucine in mixed skeletal muscle protein obtained by needle biopsy from the quadriceps muscle during a primed-continuous infusion of L-(1-13C) leucine. We used plasma 13C alpha-ketoisocaproate (representing intracellular leucine labeling) as the precursor pool for the calculation of fractional muscle protein synthesis and leucine kinetics. Fractional muscle protein synthesis was depressed in the patients with myotonic dystrophy (28% decrease, p less than 0.02). Leucine flux, leucine oxidation, and the nonoxidative portion of leucine flux were not different between the patients with myotonic dystrophy and the normal control subjects. Muscle atrophy in myotonic dystrophy reflects a selective decrease in muscle protein synthesis without any similar decrease in nonmuscle protein synthesis. This decrease may result from an impaired end-organ response to anabolic hormones or substrates.