A reevaluation of the prevalence of silicosis in the metal-mining industry of the United States, carried out by the Division of Occupational Health of the Public Health Service and the Bureau of Mines between March 1958 and September 1961, is reported. Earlier metal mine studies showed that massive dust exposures often were encountered and that few employees were free of harmful dust exposure. When dust had a high free silica content, highly exposed employees developed severe silicosis within a few years. It was common to find silicosis in 30 to 80% of employees in mines studied. In the mid 1930's, the metal mines instituted major dust control practices. World War II interrupted development of these practices as well as studies on silicosis in mines. The present reevaluation consisted first of a study of compensation and other records of official agencies to determine the magnitude of the silicosis problem. During 1950-54, 10,362 cases of silicosis were reported in 22 states. The silicotic population was primarily an older group. Of 3,455 persons with adequate employment histories available, only 10% received their entire dust exposure after 1935. Metal mining accounted for 24.5% of the reported cases. At the start of this study, it was not known whether the present prevalence of silicosis in the metal mines resulted from a reservoir of miners still working who had significant exposure before dust-control practices were instituted or was due to lack of application of dust-control practices or to inadequate standards. The environmental study included 67 underground mines employing 20,500 persons. This group represented more than 50% of the working population of underground metal mines in the U.S. The medical study included employees from 50 of these mines and a large number of uranium mines. The mines in the study represented all metals mined in commercially significant amounts in the U.S. and represented all principle mining methods. Topics include threshold limit values, particle size, a description of underground operations, dust control, ventilation, composition of mine atmospheres, history of dust sampling and comparison of methods, silicosis related to type and duration of exposure, a retrospective study of one silicosis control program, the use of the new international radiological classification of pneumoconioses in the study of silicosis, and the effects of silicosis on pulmonary function. Recommendations concerning mining practices, health supervision, and medical examinations for miners are given.