Cross-sectional osteon size was measured in undecalcified stained sections of iliac crest bone specimens from normal individuals (n = 68) and from patients with spinal osteoporosis (n = 27), primary hyperparathyroidism (n = 23), epilepsia (receiving chronic anti-convulsant therapy) (n = 11), acromegaly (n = 18), and hypothyroidism (n = 12). In each individual the shortest osteon diameter (D) and the corresponding Haversian canal diameter (d) were measured in a minimum of 20 completed secondary osteons by means of a micrometer eyepiece. Among normal males the areas of bone resorbed and formed increased with age (p less than 0.01), owing to an increase in the thickness of bone resorbed (p less than 0.01) and an unchanged thickness of bone formed. Among the females, both the areas of bone resorbed and formed decreased with age (p less than 0.05), owing to a reduction in the thickness of bone resorbed (p less than 0.05) as well as formed (p less than 0.001). Resorbed and formed areas were reduced in the epileptic (p less than 0.01) and acromegalic (p less than 0.01) groups but increased in the hypothyroid group (p less than 0.01) compared to sex- and age-matched controls. Neither the osteoporotic nor the hyperparathyroid group showed any alterations in osteon size. The Haversian canal diameter was slightly increased in the epileptic group but normal in the other patient groups. The observed changes reflect variations in the amount of work performed by osteoclasts and osteoblasts during bone remodelling and may be explained by variations in cellular activity and bone turn-over rates.