In a prospective study, 170 impacted femoral neck fractures were treated by early mobilisation and weight-bearing. The overall mortality at one year was 16%. In the 167 patients who were followed up until fracture healing or secondary instability, 143 fractures (86%) united. Instability occurred only in patients over 70 years of age, and in younger patients with a short life expectancy. Stepwise logistic regression analysis indicated that poor general health and old age (over 70 years) were risk factors. None of the other variables, such as the Garden index, Pauwels' type, and time to full weight-bearing had any influence on the development of secondary instability. Delayed operation after secondary instability caused no increase in mortality, nonunion or avascular necrosis. Functional treatment of all patients with impacted femoral neck fractures seems therefore to be justified.