The role of dietary protein in osteoporosis is unclear, with previous studies having suggested both protection and harm. The associations of total, animal, and vegetable protein with bone mineral density (BMD) and the variations in these associations with calcium intake were studied in a community-dwelling cohort of 572 women and 388 men aged 55-92 years (Rancho Bernardo, California). Multiple linear regression analyses adjusted for standard osteoporosis covariates showed a positive association between animal protein consumption, assessed by food frequency questionnaires in 1988-1992, and BMD, measured 4 years later. This association was statistically significant in women. For every 15-g/day increase in animal protein intake, BMD increased by 0.016 g/cm2 at the hip (p = 0.005), 0.012 g/cm2 at the femoral neck (p = 0.02), 0.015 g/cm2 at the spine (p = 0.08), and 0.010 g/cm2 for the total body (p = 0.04). Conversely, a negative association between vegetable protein and BMD was observed in both sexes. Some suggestion of effect modification by calcium was seen in women, with increasing protein consumption appearing to be more beneficial for women with lower calcium intakes, but evidence for this interaction was not consistently strong. This study supports a protective role for dietary animal protein in the skeletal health of elderly women.