We studied the effect of age on bone composition and osteocyte viability in femoral heads from fifty-one subjects. The assessment included determination of: bone volume, ash weight, calcium, and phosphorus content; osteocyte viability in fresh sections stained for lactate dehydrogenase activity; microfractures in fresh sections after removal of marrow elements; bone area, the presence of metabolic bone disease, and the history of microfractures in embedded calcified sections; and the extent of trabecular microfractures. Bone area and numbers of microfractures were also assessed in eight elderly hip-fracture patients. Bone volume decreased with age, but there was considerable variation in each age group, and no significant difference between men and women. Ash weight and the bone content of calcium and phosphorus also decreased with age, but were constant if corrected for bone volume. Almost all osteocytes were viable in subjects who were younger than twenty-five years, and thereafter viability progresssively decreased to a mean of 74 per cent in the eighth decade of life. There was a significant negative correlation between osteocyte viability and age. There were no evidence of metabolic bone disease in any patient. The numbers of microfractures increased with age and correlated negatively with bone viability (r = -0.31, p < 0.05); in simple linear correlation a relationship between bone area and microfractures could not be demonstrated but in multiple linear correlation, after the inclusion of bone viability, there was an additional negative correlation between numbers of microfractures and bone area (p < 0.005). Bone area and numbers of microfractures in hip-fracture patients were similar to those in age-matched controls.