Muscle fiber diameters and number of capillaries per fiber, per mm2 and around each fiber were determined in needle biopsies from the lateral part of the quadriceps muscles of 11 young women. Six subjects were untrained (UT) and five were endurance-trained athletes (ET). Average values for maximal oxygen uptake were 43.9 (UT) and 62.1 ml/kg X min (ET). Mean fiber diameter was somewhat smaller in the UT than in the ET group (39.1 and 42.7 micrometer), but the difference was not statistically significant. Fibers with the highest content of mitochondria were on the average 9 micrometer thicker than those with the lowest content. The capillary per fiber ratios were 1.11 +/- 0.07 (mean +/- S.E.) and 1.69 +/- 0.13 in the UT and ET groups, respectively. The number of capillaries around each fiber was 3.04 +/- 0.17 (UT) and 4.42 +/- 0.31 (ET). After correction for shrinkage the number of capillaries per mm2 was 301 (UT) and 404 (ET). The capillary per fiber ratio increased with increasing fiber diameter and with the content content of mitochondria.