Two hypotonic boys, aged 7 years, 3 months and 7 years, 7 months, who possessed sufficient speech to demonstrate a severe thought disorder and who differed markedly in their activity levels, were subjected to a biopsy of the quadriceps muscle. The biopsies revealed atrophy of type 2 muscle fibers, as well as variability in the size of these fibers. The findings could be compatible with a denervation phenomenon.