We describe the clinical presentation, course, pathologic findings, and biochemical abnormalities found in three adult siblings with an idiopathic lipid storage myopathy. The major presenting symptoms were weakness and cramping, which were profound in one patient, moderate in the second patient, and mild in the third. All three individuals exhibited true myotonic discharges on EMG, normal or mildly depressed muscle free carnitine levels, and borderline delayed ketosis (40 hours) with fasting. Muscle biopsies from all three showed neutral lipid storage. Polymorphonuclear leukocytes and macrophages were vacuolated. No systemic abnormalities of glucose or fat metabolism were identified at rest, with fasting, or with exercise. The two more severely affected patients have responded to medium-chain triglyceride diet and oral carnitine with increased strength and muscle bulk and decreased histochemically observed neutral lipid stores in muscle. One patient has had a resolution of the electrical myotonia. Prednisone therapy in one patient resulted in a loss of cramping sensations but not improvement in strength. We suggest that some cases of idiopathic lipid storage myopathy may be safely and effectively treated with carnitine and medium-chain triglyceride diet.