At the request of the Mine Safety and Health Administration (MSHA), the National Institute for Occupational Safety and Health (NIOSH) recently investigated the health of workers at two silica flour mills. Both mills have a long history of failure to maintain workplace concentrations of dust containing free silica below the MSHA exposure standard. Thirty-seven percent of sixty-one workers and ex-workers with one or more years of silica dust exposure had chest radiographic evidence of silicosis. Because of the high prevalence of silicosis in workers at these two silica flour producers, MSHA silica dust measurements were reviewed for twenty-five other active U.S. silica flour producers. Eighty-nine percent of the work force in these 27 silica flour producers are employed at workplaces where more than twenty-five percent of the dust samples reviewed were above the exposure standard. Fifty-three percent of all samples were above the MSHA standard. No significant decline in silica dust levels occurred during the period (1974-1979) covered by the data supplied by MSHA. Based on the NIOSH study and MSHA data, it is apparent that the risk of silicosis is very high among workers in this industry. Prevention of this disease will require a concerted effort of government enforcement agencies, workers and management.