Biopsy samples of adipose tissue from the upper thigh were collected in spring and in late summer/autumn from 370 wild polar bears (Ursus maritimus), including adult females with and without cubs, adult males and juveniles. Mean adipocyte volume was measured from all samples and chemical assays of the lipid, total protein and collagen were also performed on samples from 53 bears. Mean adipocyte volume was smaller in all specimens in spring than in late summer/autumn, but the differences were greatest for solitary adult females. The range of adipocyte volumes was much greater for adult females than for adult males, and in females only, mean adipocyte volume correlated significantly with total body mass. Therefore, adipocyte volume measurements from biopsy samples provide some information about fatness in adult females, but are worthless as an indicator of body composition in males and juveniles. In juveniles and females, but not adult males, the lipid content of the adipose tissue was up to 18% lower in autumn than in spring. The collagen content was significantly higher in autumn than in spring in all bears except females with cubs. We suggest that these differences in chemical composition arise from accumulation of water within and between the adipocytes, which would minimize tissue shrinkage, and from changes in the vascularization of the adipose tissue. These properties may be adaptations to rapid fattening and prolonged fasting and the sex differences may reflect the contrasting reproductive strategy of female and male polar bears.