The incidence of microbes in the nasal cavities of workers in three paper and board mills was investigated. A total of 234 persons exposed to microbial aerosols and splashes from paper machine wires and debarker drums formed the exposed group. The control group consisted of 294 workers from the dry working areas: the winding and packing sections. Chi-square analysis was used to test the differences in the frequency of microbial incidence and various symptoms between the exposed and control groups. The nasal cavities of many workers, particularly workers in the debarkers, proved to be contaminated by Klebsiella pneumoniae, other coliforms, yeasts, and molds; usually only one microbe was involved, but sometimes two or several species were found. Nasal bacteria and yeasts were largely derived from the mill and debarker air; the microbes in the air came mainly from process waters. Lack of association of nasopharyngeal symptoms with either exposure to aerosols or nasal microbial contamination was interpreted as an indication of host defenses that were adequate to protect workers from harmful microbial colonization in paper mill environments.