Osteoporosis is an accepted fact of the female's aging process. This decrease in bone mass is most often linked to a lack of estrogen; however, prolonged inactivity is also known to cause atrophy in both muscle and bone. The purpose of this study was to investigate the relationship between the degree of osteoporosis and the physical activity levels of a group of post-menopausal women. Forty women between the ages of 60-69 yr were selected at random from a group of volunteers. The possible existence of a correlation between cortical diameter and activity level was investigated by using linear-model analysis. The width of the cortex may be related to many factors in addition to activity level. Some of the more important factors also considered were height, weight, ponderal index, age, years since taking estrogen, and total metacarpal diameter. The inclusion of these terms in the linear-model analysis allowed for an accounting of variability as associated with variables that could not be held constant. The variables showing the most significant relationships with the cortical diameter were the activity level and the use of estrogen. The relationships investigated through the linear-model analysis do not need to be independent. It is possible that combinations of these variables act to retard or enhance osteoporosis. Therefore, because the primary concern of the investigators was the relationship between physical activity and osteoporosis, a t-test was performed comparing the 10 most active women with the 10 most sedentary.(ABSTRACT TRUNCATED AT 250 WORDS)