Capillarization of skeletal muscle has been reported to be both maintained and reduced with advancing age. This conflict may represent methodological differences between biopsy studies. We have examined capillarization throughout two muscles, soleus and extensor digitorum longus (EDL), from a well-established colony of aging mice, and related this to fiber number (C/F ratio) and type. Labeling of muscle capillaries was performed with the biotinylated Griffonia (Bandeiraea) simplicifolia lectin (GSL 1) using immunochemistry. The results showed a significant increase in the C/F ratio in the aged mice when compared with the younger (6-month mice soleus = 1.296, 95% CI 1.226-1.366 vs 28-month mice soleus = 1.530, 95% CI 1.488-1.572, p <.001; 6-month mice EDL = 0.881, 95% CI 0.751-1.011 vs 28-month mice EDL = 1.124, 95% CI 1.028-1.220, p = .017). These differences could not be accounted for by changes in fiber type but may reflect loss of fibers. Alternatively, there may be increased angiogenic drive or a failure of downregulation of angiogenesis.