The purpose of the study was to investigate if a questionnaire sent by mail can give a reliable assessment of outcome on the Glasgow Outcome Scale. A questionnaire was developed for the Glasgow Outcome Scale (GOS) and a second questionnaire for the Extended Glasgow Outcome Scale (GOSE). The questionnaires were self-contained and designed to be completed by either a head-injured person or a proxy. The questionnaires were studied in two ways: each questionnaire was administered twice (at an interval of approximately 2 weeks), and ratings from the postal questionnaires were compared to ratings from a structured interview conducted by telephone. The four studies were carried out in separate groups of head-injured participants consisting of 32-38 individuals. For the test-retest comparison, k(w) (quadratic weights) was 0.94 for the GOS questionnaire and 0.98 for the GOSE questionnaire. For the comparison with the telephone interview, k(w) was 0.67 for the GOS and 0.92 for the GOSE. The values of k(w) indicate good agreement for all comparisons. We conclude that it is possible to obtain reliable outcome data after head injury using postal questionnaires.