A controlled clinical comparison of 6 and 8 months of antituberculosis chemotherapy in the treatment of patients with silicotuberculosis in Hong Kong. Hong Kong Chest Service/tuberculosis Research Centre, Madras/British Medical Research Council. Patients with silicotuberculosis have been reported to respond poorly to antituberculosis chemotherapy. Therefore, in a study in Hong Kong, 240 Chinese male patients with both silicosis and pulmonary tuberculosis were all prescribed treatment three times weekly with streptomycin, isoniazid, rifampin, and pyrazinamide, allocated at random to be given for a total duration of either 6 (M6 regimen) or 8 months (M8 regimen) in a concurrent comparison. Those with a history of previous antituberculosis chemotherapy received ethambutol as well for the first 3 months. The intake in the M6 regimen was terminated when preliminary results showed that it was inadequate, and a further 53 patients were assigned to the M8 series. Of 91 assessable patients in the concurrent comparison with susceptible strains pretreatment, 44% were culture negative at 1 month, 80% at 2 months, and 98% at 3 months, and 1 had an unfavorable bacteriologic response during chemotherapy. During 3 yr of assessment, bacteriologic relapse after chemotherapy occurred in 22% of the M6 compared with 7% of the M8 patients (p less than 0.025, log-rank test). Inadequate chemotherapy was received by 12% of the 240 patients in the concurrent comparison because of default and by 22% because of adverse effects, but by 3 yr 92% of patients with susceptible strains pretreatment in each series had a favorable status following retreatment for relapse or for initially inadequate chemotherapy when required. The results show that patients with silicosis require at least 8 months of treatment. 