We studied 50 patients with the merosin-positive form of congenital muscular dystrophy (MP-CMD) clinically and pathologically. The frequency of MP-CMD in our laboratory was approximately one-half that of the Fukuyama type and one-sixth that of Duchenne muscular dystrophy. The early signs of MP-CMD included decreased fetal movement during pregnancy (14%) and poor suck (42%), floppiness (30%), and respiratory difficulty (16%) in early infancy. Eighty-six percent of the patients had delayed motor development. Ninety-two percent of the patients followed beyond age 4 years had learned to walk. The disease was relatively slowly progressive, except in six patients who rapidly lost ambulation. Almost all patients had normal IQ, except four who were mildly to moderately retarded. Of the patients examined by cranial CT/MRI, 24% showed cerebral atrophy and 11% had areas of white matter lucency. Muscle biopsy results in those younger than 5 years showed mild dystrophic changes consisting of variation in fiber size and scattered necrotic and regenerating fibers. In older children, there were additional chronic dystrophic changes, including fiber splitting (32%), moth-eaten appearance (32%), marked fatty replacement (46%), and abnormal fiber type distribution (59%). The manifestations of MP-CMD were generally milder and more slowly progressive than those of the Fukuyama type and merosin-negative form of congenital muscular dystrophy.