From October to December 1985, 78 patients with active pulmonary tuberculosis (mean age, 73.4 years) were given the Mantoux test (1 unit). Non-reactors were rechallenged with 10 units. The results of the test group were compared with those of 108 controls. Fifty-three patients with radiologically inactive tuberculosis (mean age, 71.4 years) were included. Twenty-one had radiologically inactive tuberculosis without prior treatment, 32 had radiologically inactive tuberculosis with a mean interval of 11.9 years from previous treatment, and 55 had no radiological or clinical evidence of tuberculosis (mean age, 72.0 years). The Mantoux test in either strength showed either low sensitivity or specificity for active tuberculosis. Although both showed good positive predictive value of over 80% in indicating an abnormal chest X-ray, further investigations were required to establish disease activity. We concluded that the Mantoux test has limited usefulness, as a screening or diagnostic tool for active tuberculosis, in the elderly population of Hong Kong, where the prevalence of tuberculosis is high.