Metabolic bone disease in laying fowl has been associated with mortality and decreases in egg production. Recently there appears to have been an increasing incidence of the disease and the affected flocks have shown a variable response to dietary treatments. In 14 birds from eight recent submissions, avulsion of the patellar ligament together with part of its bony insertion was observed. Because the skeletons of the affected birds were judged from radiographs to be osteopenic, the lesions were probably pathological fractures. Histopathological studies of four birds revealed osteoporosis affecting cancellous bone trabeculae; in one bird, however, the medullary bone trabeculae were largely composed of unmineralized or poorly mineralised matrix, a finding which can be interpreted as osteomalacia.