1. During a 15-month period, 110 elderly patients admitted with fractured neck of femur were studied in comparison with 72 elderly control patients undergoing elective surgery admitted over the same period to the same orthopaedic wards. 2. A striking finding was the marked similarity of all variables measured in the fracture and control groups. The principal differences between the two populations were that the fracture patients had a lower mean forearm bone mineral content, and that their serum concentrations of albumin, globulin and phosphate were reduced. 3. Neither measurements of radioisotopic calcium absorption, nor those of the serum concentrations of calcium, 25-hydroxy-vitamin D 3 nor alkaline phosphatase provided satisfactory discrimination between the groups. 4. Several indices were devised, based on linear combinations of the test results obtained, in an attempt to predict the liability to future fractures of patients being considered for prophylactic therapy with oestrogens or other drugs. However, further work is required to define an index of improved predictive power and to evaluate it prospectively.