A survey of size-fractionated dust exposure was carried out in 10 wood processing plants across the United States as part of a 5-year longitudinal respiratory health study. The facilities included a sawmill, plywood assembly plants, secondary wood milling operations, and factories producing finished wood products such as wood furniture and cabinets. Size-fractionated dust exposures were determined using the RespiCon Personal Particle Sampler. There were 2430 valid sets of respirable, thoracic, and inhalable dust samples collected. Overall, geometric mean (geometric standard deviation) exposure levels were found to be 1.44 (2.67), 0.35 (2.65), and 0.18 (2.54) mg/m, for the inhalable, thoracic, and respirable fractions, respectively. Averaged across all samples, the respirable fraction accounted for 16.7% of the inhalable dust mass, whereas the corresponding figure for thoracic fraction as a percentage of the inhalable fraction was 28.7%. Exposures in the furniture manufacturing plants were significantly higher than those in sawmill and plywood assembly plants, wood milling plants, and cabinet manufacturing plants, whereas the sawmill and plywood assembly plants exhibited significantly lower dust levels than the other industry segments. Among work activities, cleaning with compressed air and sanding processes produced the highest size-fractionated dust exposures, whereas forklift drivers demonstrated the lowest respirable and inhalable dust fractions and shipping processes produced the lowest thoracic dust fraction. Other common work activities such as sawing, milling, and clamping exhibited intermediate exposure levels, but there were significant differences in relative ranking of these across the various industry segments. Processing of hardwood and mixed woods generally were associated with higher exposures than were softwood and plywood, although these results were confounded with industry segment also.