Muscle biopsies of 9 children with congenital myotonic dystrophy were taken during the first year of life. All but one biopsy were abnormal showing nonspecific changes like an increased variability of fiber size, a selective atrophy of type I or type II fibers or an increased number of intrafusal muscle fibers. Nevertheless correct diagnosis of the disease based on morphological data only is impossible in some cases. A maturational arrest of the muscle is not a obligate feature of the disease. We conclude that muscle biopsy should be postponed after the first year of life.