Immunoreactivity for the intermediate filament proteins vimentin and desmin was assessed in the type 2 fibers of biceps brachii muscle biopsies from 20 adults with myotonic dystrophy (MD). Some type 2C fibers showed strong reactivity with both anti-vimentin and anti-desmin, suggesting that they were regenerating fibers. Some type 2A and 2B fibers of various diameter were weakly reactive with anti-vimentin, but none of the type 1 fibers were reactive with anti-vimentin. Hardly any reactivity with anti-vimentin was demonstrated in patients in the younger age group (10-29 years) with mild illness, but vimentin-positive type 2 fibers increased in the age groups 30 years of age and older. The mean diameters of type 2A and type 2B fibers in vimentin-positive patients were larger than in vimentin-negative patients. In the patients in the 30 years of age and older group, the vimentin-negative patients had more severe muscle weakness than the vimentin-positive patients. These findings suggest a possible relationship between the process of regeneration in type 2 fibers in adult MD patients and the increase in their mean diameter.