1. Eleven patients with the bone loss of ageing were treated with the vitamin D analogue 1 alpha-hydroxycholecalciferol and calcium for 3--6 months. 2. Muscle biopsies were taken from the vastus lateralis before and after the treatment and the activity of several enzymes was measured. Succinate dehydrogenase and total phosphorylase activities, which are a measure of the oxidative capacity, were low and increased significantly with the treatment. The lactate dehydrogenase activity, which can be taken as a measure of the anaerobic metabolism, was normal and did not change with treatment. The phosphagen stores, ATP and creatine phosphate were low and increased to normal with treatment. 3. Histochemical classification of the fibre composition revealed that the treatment induced an increase in the relative number of fast-twitch a (FTa or type II A) fibres accompanied by a reduction of the fast-twitch b (FTb or type II B) fibres. The cross-sectional area of the FTa fibres also increased with the treatment. 4. The present findings indicate that treatment with the active vitamin D analogue, 1 alpha-hydroxycholecalciferol, and calcium improves the myopathy associated with the bone loss of ageing.