In order to investigate the inheritance in congenital nemaline myopathy (CNM), we studied the family histories and pedigrees of 13 patients with CNM from 10 families, and the 20 patients, by physical examination, single fibre electromyography, ultrasonography of muscles, measurement of serum creatine kinase, muscle biopsy, and electrophoresis of muscle proteins. None of the parents was affected. In three families there were two affected children. Of the parents, 15 showed deficiency of type 2B muscle fibres, and all except one father showed some other minor neuromuscular abnormality. These may represent heterozygous manifestations of recessive gene. Most of the ancestors came from sparsely populated rural communities in the west of Finland. We conclude that in the Finnish CNM patients, the mode of inheritance appears to be recessive. Apart from a few instances of dominant inheritance, most cases published also seem compatible with recessive inheritance.