Atraumatic fractures caused by osteoporosis may be a serious complication of primary biliary cirrhosis. Mean (+/- S.D.) bone mineral density in the lumbar spine in 210 ambulatory women with primary biliary cirrhosis was 1.02 +/- 0.19 gm/cm2, 7% lower than that in 139 age-matched normal women (after adjustment for age and body weight) (p less than 0.001). Bone mineral density in the lumbar spine was inversely related to a risk score index of liver disease severity (r = -0.29, p less than 0.001). The mean rate of bone loss in 105 of these 210 women was 2%/yr +/- 4%/yr, twice as great as in the 139 normal women (p less than 0.02). In 20 women with primary biliary cirrhosis followed up after orthotopic liver transplantation, bone mineral density in the lumbar spine decreased at 3 mo (p less than 0.01), and this decrease may have resulted in atraumatic fractures in 13 of them. Bone mineral density in the lumbar spine then increased (p less than 0.01) so that by 12 mo the median bone mineral density in the lumbar spine was similar to that before transplantation and by 24 mo it was 5% above it. Therefore we conclude that the progressive bone loss observed in primary biliary cirrhosis (which is further accentuated immediately after transplantation) may be halted, and the bone mass may be restored toward normal within 2 to 3 yr after orthotopic liver transplantation.