Due to the lack of epidemiologic data on osteoporosis in the young, we identified all 22 Olmsted County, MN, residents aged 20-44 years when first diagnosed with established osteoporosis in 1976-1990. The overall age- and sex-adjusted incidence rate was 4.1 per 100,000 person-years (95% CI 2.4-5.9) with a female to male ratio of age-adjusted rates of 1.2:1. The majority represented secondary osteoporosis (12 steroid-induced, 3 postmenopausal, 2 delayed puberty, 2 anticonvulsant-induced, 2 gastrointestinal disease, 2 alcoholism, 1 anorexia nervosa, and 7 other etiologies; some individuals had more than one factor present) but two had idiopathic osteoporosis (incidence 0.4 per 100,000 person-years, 95% CI 0-0.9). To further characterize the patients with idiopathic osteoporosis, we also reviewed the entire Mayo Clinic experience with such patients from 1976 to 1990, regardless of residency. A total of 56 patients (30 female/26 male) were identified with a median age at diagnosis of 34 years. Only 8% were hypercalciuric at presentation. There was a preponderance of cancellous bone fractures (vertebral 81%, rib 37%, wrist 13%), although 13% did have hip fractures. Transiliac bone biopsies were available in 18 patients. As compared to age- and sex-matched controls, the osteoporotic subjects had a significant reduction in trabecular bone volume, cortical thickness, and mean wall thickness, the latter suggesting an abnormality in osteoblast function in these individuals.(ABSTRACT TRUNCATED AT 250 WORDS)