BACKGROUND Percutaneous Knowles' pinning is a well accepted treatment of undisplaced femoral neck fracture, but there are some complications that have seldom been discussed. METHODS One hundred and eighty two patients with undisplaced femoral neck fractures, at a mean age of 67 years, were treated by percutaneous Knowles' pinning between 1983 to 1989, and followed for an average of 102 months. The clinical results and complications were evaluated. The data were collected on the basis of age and bone density. Student's t-test was used to evaluate the significance of the results. RESULTS The mean union time was 20 weeks. In all the patients, 85.8% had good results, 3.8% patients had acceptable results and 10.4% patients had poor results. Old age (> or = 65 years) and osteoporosis (Singh index < or = 3) were two factors of the poor result. Thirty-two cases (17.6%) developed complications. The incidence of nonunion or implant problems was apparently higher in the older or osteoporosis group, but avascular necrosis of femoral head correlated little with old age or osteoporosis. CONCLUSIONS Although percutaneous Knowles' pinning is simple, safe, economic and reasonably effective for the treatment of undisplaced femoral neck fractures, we should pay more attention to the uncooperative, old, and osteoporotic patients.