The aim of the study was to describe initiation of osteoporosis drug therapy after bone mineral density (BMD) testing and to determine any association with BMD test results obtained, physician factors, or both. The setting was the Kaiser Foundation Health Plan (KFHP), a large health maintenance organization (HMO) in Northern California. Data were collected from bone densitometry centers at four KFHP medical centers sites in Sacramento, San Rafael, Fresno, and Oakland. We identified 17,290 women aged > or = 45 years who had BMD testing between January 1, 1997 and June 30, 1999. After excluding those for whom any osteoporosis drugs were prescribed in the year before testing, 8020 women were available for analysis. Using logistic regression, we examined the association between BMD diagnosis (i.e., osteoporosis or osteopenia versus normal) and initiation of drug therapy, for osteoporosis (including hormone replacement therapy (HRT), alendronate, etidronate, raloxifene and calcitonin) within 6 months after the test. Among the 8020 women, 1934 (24%) filled a prescription for an osteoporosis drug within 6 months after BMD testing. Compared with women who had a normal BMD test result, women diagnosed with osteopenia were nearly 4 times more likely (OR = 3.7; CI = 3.0-4.4), and women diagnosed with osteoporosis were 15 times more likely (OR = 15.0; CI = 12.5-18.1), to fill a prescription for an osteoporosis drug within 6 months after BMD testing. Women with high exposure to corticosteroid agents were twice as likely (OR = 2.1; CI = 1.7-2.7) to start osteoporosis drug therapy compared with women who were not similarly exposed; women diagnosed with recent osteoporotic fractures were 50% more likely (OR = 1.5; CI = 1.2-1.9) to begin therapy than women without such fractures. Despite the strong association between BMD and initiating treatment, nearly half the osteoporotic women did not initiate treatment. In addition, we found that age strongly influenced choice of osteoporotic drug. Compared with osteoporotic women aged 45-54 years, women aged 55-64 years who started drug therapy were 40% more likely (OR = 1.4; CI = 1.0-2.2) and women aged > or = 65 years were twice as likely (OR = 2.0; CI = 1.4-2.8) to start non-HRT drugs. BMD test results indicating osteoporosis were thus strongly associated with increased likelihood of beginning drug therapy, and half of such women initiated therapy. Drug initiation was also associated with other factors, including age, use of corticosteroid agents, recent fracture, and physician characteristics. However, these factors showed much weaker associations than those found for BMD. Health care providers must consider whether test results will influence treatment decisions, and our data indicate that results of BMD testing do influence management decisions regarding osteoporosis drug use for women.