We reviewed clinical data and quantitative muscle biopsy data from infants with hypotonia and developmental delay who had relatively small type 1 muscle fibers (fiber size disproportion; FSD). Study material consisted of 49 thigh muscle specimens from 37 patients, 1 week to 11.5 years old, who had FSD diagnosed between 1971 and 1985. Beyond 2 months of age, FSD was greater than 12% in at least one biopsy from all but 2 patients. In sequential specimens obtained from 11 patients, we observed that FSD may resolve, persist unchanged, or intensify with age. In 6 patients, hypotonia regressed, and development tended to normalize during 1-4 years of observation; FSD resolved in 4 cases, but numerical predominance of type 1 fibers persisted in 3 of them. In a small group designated congenital myopathy with FSD only, the disproportion in fiber diameter persisted unchanged or intensified with time and was more severe after the age of 6 months than in the other subgroups. We classified FSD as definitely or probably myopathic (15 cases), definitely or probably neuropathic (17 cases), and benign maturation delay (5 cases). Six of 17 neuropathic cases had a history of perinatal asphyxia. In most cases, type 1 fibers were absolutely small and type 2 fibers were hypertrophied, independent of classification. FSD is a manifestation of muscle maturation delay of variable etiology. The prevalence of nervous system disorders in our series suggests that neuronal dysfunction plays an important role in pathogenesis. We were unable to define morphometric criteria that consistently distinguished congenital myopathy with FSD from definable neuropathic FSD or from benign maturation delay.