A previously healthy 12-year-old boy developed pain on walking and x-rays showed osteoporosis. Over the next 2 years deterioration occurred, the condition became extremely severe, and he was confined to a wheelchair. After 5 years, marked kyphoscoliosis and pigeon chest deformity were present and little increase in height occurred. A wheelchair accident at the age of 17 resulted in several major long bone fractures. Iliac crest biopsies were taken at ages 15 and 17, and subjected to quantitative histology. A histochemical technique for osteoclast recognition by acid phosphatase activity showed resorption parameters to be normal. Double tetracycline labeling and histochemical identification of osteoblasts showed no abnormality of endosteal bone formation. Because of "coupling" of endosteal formation and resorption, these measurements might primarily reflect bone turnover. Failure of periosteal bone formation as shown by failure of radial growth of long bones and of epiphyseal growth was clearly evident. It is likely that osteoporosis developed in this patient due to a reduction in bone formation of unknown etiology rather than by increased bone resorption.