A prevalence of 0.11 % of focal epithelial hyperplasia (FEH) was found among 20, 333 adult Swedes. There was no sex difference, the lesion was most prevalent in age groups above 45 years and the lesion was most frequent on the tongue. The frequency of FEH in 15, 132 consecutive routine biopsies was 0.26 %. Four FEH-cells were ultracstruturally examined. They exhibited a clear cytoplasm with scattered ribosomes, a peripheral condensation of tonofilaments, a central aggregation of chromatin clumps with loss of nuclear membrane and an accumulation of desmosome fragments. No viral particles could be identified in these FEH-cells.