The pathologic features of muscle and/or spinal cord were studied in 96 infants and children with contractures of multiple joints (arthrogryposis multiplex congenita), usually in association with other congenital abnormalities. Ninety of these infants had a neurogenic form of arthrogryposis, and six had primary muscle disease. The neurogenic form, unlike the myopathic form, was usually associated with other congenital abnormalities. The most frequently associated congenital changes were low-set ears, micrognathia, wide flat nose, short neck, congenital heart disease, high-arched palate, hypoplastic lungs, and cryptorchidism. Some of the associated abnormalities could be attributed to muscle weakness, occurring during intrauterine development. A variety of skeletal muscle changes were observed, including primary myopathic alterations, fiber type predominance and disproportion, hypoplasia, aplasia, and denervation atrophy. When the primary alterations were in the spinal cord, abnormalities of anterior horn cells of several distinct types were recognized--absence of cells, diminution, dysgenesis, degeneration, and axonal reaction. The changes in anterior roots corresponded to those of the anterior horn cells.