The effect of inactivity was examined in muscle biopsies from ten patients who were operated upon for unilateral knee joint lesions of long duration. Biopsies were taken from the vastus medialis on both sides, and transverse frozen sections were stained for myofibrillar adenosine triphosphatase (ATPase). Fibre type counting and exact measurements of muscle fibre size were performed and compared on the two sides. The muscle fibres on the affected side showed a definite reduction in size and, on the average, both types of fibres were about equally affected (27.6% vs. 25.0% reduction for types 1 and 2). However, there were considerable and unpredictable variations from case to case with predominant affection of type 1 in some and type 2 in others. The numerical distribution of fibre types did not change. It is concluded that both fibre types are affected in disuse atrophy, but it seems impossible to predict which of the fibre types will show the most severe atrophy. Thus, enzyme histochemical staining methods are of little help in the diagnosis of disuse atrophy.