This abridged account of a report to the British Medical Research Council describes a long-term investigation of 1,503 subcapital fractures of the femur, almost all of which were treated by reduction and internal fixation. With three exceptions, union occurred in all Garden Stage I and Stage II fractures and in 67% of Stage III and Stage IV fractures, of which only 14-5% were united at six months. In women, late segmental collapse was seen after union had occurred in 16% of Stage I and in 27-6% of Stage III and Stage IV fractures. Delay of up to one week before operation had no significant effect on the incidence of non-union or of late segmental collapse. The incidence of union followed by late segmental collapse was higher in women with normal bone density than in those with osteoporosis. Smith-Petersen nailing was found to be the least effective form of fixation in displaced fractures. The age and physical state of the patient, the accuracy of reduction, and the security of fixation had the greatest influence on union.