We present 11 patients with congenital fiber type disproportion suggesting the existence of two different clinical groups. The first group not associated with other diseases, presents a uniform clinical picture and a generally good prognosis, although the patients with severe respiratory involvement can die. The second group includes the cases in which CFTD is associated with other congenital diseases such as Lowe's Syndrome, Möbius' Syndrome, hypothyroidism and hydrocephalus. In this group the clinical presentation and prognosis is that of the associated disease added to that of CFTD. The frequent association of CFTD with other congenital diseases suggests that CFTD may not be a specific myopathy but a histological abnormality due to different pathogenic insults which interfere with the normal growth and maturation of the muscle fibres.