This paper presents estimates of airborne fiber concentrations and fiber size for European man-made mineral fiber (MMMF) factories on the basis of measurements made in 1977-1980. The airborne fiber concentrations previously reported at a conference of the World Health Organization (WHO) and the International Agency for Research on Cancer in 1982 have been revised to harmonize the results with the WHO-European MMMF reference counting level. The result was an approximate doubling of the reported airborne fiber levels. After the revisions the average combined occupational group concentrations in the rock- and glass-wool plants were still generally low (less than 0.01 fibers/ml). In the glass continuous-filament factories the airborne fiber concentrations were very low (less than 0.01 fibers/ml). The average plant median for fiber length ranged from 10 to 20 microns, and the corresponding median diameters ranged from 0.7 to 2 microns. In general the glass-wool fibers were thinner than the rock-wool fibers. The fiber concentrations measured in other studies in the MMMF production and user industries are reviewed. Higher levels (between 0.1 and 1.0 fibers/ml) have been measured in some insulation wool production, secondary production, and user industries. The highest levels (greater than 1.0 fibers/ml) occurred in very fine glass-fiber production and in other specialist insulation wool usage.