Muscle fiber dimensions were studied in six newly diagnosed insulin-dependent young diabetic males and in four age, sex and weight matched controls. Three of the patients had never received any antidiabetic medication before biopsy and the others had been treated with insulin for a few days only. Mean fiber caliber of the anterior tibial muscle was smaller in diabetics than in controls, the values being 61.8+/-4.3 micron (SD) versus 77.8+/-9.1 micron (2p less than 0.01). The size-frequency distribution of fiber calibers showed that a considerable part of the fiber population was reduced in size. No significant changes were found in sarcomere length, number of muscle or interstitial cell nuclei or number of capillaries. The observation of an over-all reduction of muscle fiber size following a few weeks of severe hyperglycemia is in accordance with previous experimental studies. We conclude that the catabolism associated with a sustained and uncorrected diabetic state in man leads to muscle fiber atrophy.