We studied a family in which seven individuals in three generations had slowly progressive weakness without atrophy, myalgia, cramps, or episodic weakness. Creatine kinase was normal, and EMG showed only slight “Inyopathic” changes. Neuromuscular transmission was undisturbed. Muscle biopsies were performed in three patients. About 60 to 90% of all fibers contained tubular aggregates. There was a marked variation in fiber size and a marked type I1 fiber atrophy. Biopsy of an asymptomatic family member was normal. The nature of the underlying disease was obscure.