Satellite cells were examined qualitatively and quantitatively in muscle biopsies from children when malnourished, during nutritional rehabilitation, and after clinical recovery. The proportion of satellite cell nuclei relative to myonuclei was significantly lower in malnourished subjects than in well-nourished age-matched controls (4.5 +/- 1% vs. 8.1 +/- 1.0%). The proportion of satellite cells remained low during the early period of "catch-up growth" but was significantly increased in the recovered subjects (10.5 +/- 1.0%). Satellite cells were small and their nuclei were heterochromatic in biopsies from the malnourished subjects. The cells were often partially segregated from the parent fiber by an external lamina. In recovering and recovered subjects many of the satellite cells enlarged, and the appearance of their nuclei and cytoplasmic organelles suggested a more active state. Intervention of external lamina between the satellite cell and the myofibre was uncommon in the recovered subjects.