In a morphometric study on biopsied muscles from 5 patients with nemaline myopathy (NM) and 5 with congenital fiber type disproportion (CFTD), the common findings were relative type 1 fiber smallness, type 1 fiber predominance and occasional hypertrophic type 2 fibers. In NM, the relatively larger type 1 fibers increased in number with age in parallel with a decrease in the number of normal to hypertrophic type 2 fibers, reflecting active fiber type transformation from type 2 to type 1, which resulted in striking type 1 fiber predominance. The presence of scattered non-atrophic type 2C fibers also reflected active fiber type transformation because the fibers during the maturational or degenerating process are known to show the type 2C reaction on ATPase staining. On the other hand, the type 1 fibers in CFTD were small in caliber and showed minimal variation in size, suggesting practically no fiber type transformation from hypertrophic type 2 to type 1.