To determine if the severity of osteoporosis and its resultant hyperkyphosis cause measurable impairment of lung function, 74 women referred for osteoporosis evaluation underwent pulmonary function testing. Women with thoracic wedge compression fractures secondary to osteoporosis had significantly lower percent predicted FVC than did those without fractures. In hierarchical regression analysis, after controlling for age and arm span, there was a significant effect on FVC of the degree of hyperkyphosis as measured by Cobb's angle (increment in R2 = 0.14, p less than 0.001). The addition of the number of vertebral fractures to the model was also significant (increment in R2 = 0.06, p less than 0.002), but cortical bone volume, bone mineral density, and smoking status did not significantly improve the model. From one half of subjects tested, a regression equation was generated relating %FVC and the number of thoracic fractures: %FVC = 103.4 - 9.4 x number of fractures. When used to predict lung function impairment in the second half of study subjects, the correlation between measured and %FVC was r = 0.59 (p less than 0.002). Kyphosis and thoracic compression fractures caused by osteoporosis produce modest but predictable declines in vital capacity in women.