From 1975 to 1984, the incidence of cancer was determined among workers employed in a French man-made mineral fiber (MMMF) production plant. The cohort, including 1 374 fully active or retired men, represented 12 793 person-years. Forty-one patients had cancer, 19 of which were in the upper respiratory and alimentary tract and five of which were lung cancers. The number of lung cancers observed did not significantly differ from that expected [standardized incidence ration (SIR) 0.7]. However the incidence of upper respiratory and alimentary tract cancers was significantly higher (SIR 2.2), especially for those in the larynx (SIR = 2.3), pharynx (SIR 1.4), and buccal cavity (SIR 3.0). The relative risk of cancers in the upper respiratory and alimentary tracts seemed to increase with the length of exposure to glass fibers and exceeded the value of 3.0 for more than 10 years of exposure. This result does not seem to be attributable to an excess of tobacco and/or alcohol consumption. No previous survey of the MMMF industry has shown such a risk, except in Italy. Thus this finding has to be confirmed.