We collected 245 adipose tissue biopsies from adult polar bears Ursus maritimus in north-eastern Manitoba during the course of long-term population studies between fall 2001 and spring 2004. In summer, the sea ice of Hudson Bay melts completely and the entire polar bear population is forced to fast on land for c. 4 months. During this period, the adipose tissue of females contained significantly more lipid than that of males, consistent with preparation for pregnancy and lactation. The adipose tissue of females with cubs contained less lipid than that of solitary females, likely reflecting greater mobilization of lipid during lactation. Although most of the population returns to the sea ice to hunt after freeze-up in mid-November, pregnant females enter maternity dens where they continue to fast for an additional 4 months. As a result, the adipose tissue of females emerging from dens at the end of this 8-month ‘reproductive fast’ contained significantly less lipid than females in the fall. There was also evidence of a decline in the adipose tissue lipid content of females emerging from dens over the course of the study. Although this trend was based on limited sample sizes, it suggests that the overall condition of new mothers may be declining. Fat biopsies collected from 20 adult polar bears during a mark–recapture survey on the winter sea ice of south-eastern Beaufort Sea showed that the fatty acid (FA) composition of the superficial adipose layer was largely uniform with depth; however, lipid content significantly increased from skin to muscle. Finally, adipose tissue collected from the belly, rump and baculum depots of bears killed by native subsistence hunters showed no site-specific differences in either FA composition or lipid content. These data suggest that a single sample from any large superficial depot will accurately reflect a polar bear's total superficial adipose store. We suggest that lipid content of adipose tissue may provide valuable information on changes in polar bear condition in response to changes in arctic climate and prey distribution.