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Notes: The purpose of this investigation was to study stereologically the histopathologic alterations occurring during a human experimental gingivitis, and to establish a relationship between clinical parameters and histologic findings. Eight dental students volunteered for the study. After a prophylaxis they performed optimal oral hygiene for 3-4 weeks to reach mean plaque and gingival indices approaching zero. They then abandoned all oral hygiene procedures for a period of 21 days. At d 0, 4, 7, 14 and 21, Plaque Index (PII), Gingival Index (GI) and Gingival Exudate Flow Rate (GEFR) were assessed, and a buccal biopsy of their gingiva was taken. Point counting procedures were performed at 2 different levels of magnification to estimate the volume densities of epithelium, infiltrated and non-infiltrated connective tissue, and collagen. The percentages of polymorphonuclear neutrophilic granulocytes, lymphocytes, plasma cells, macrophages and fibroblasts were estimated by counting the number of profiles of these cells in the connective tissue area close to the apical end of the junctional epithelium. The histological picture during the entire experiment was one of an early lesion (Page & Schroeder 1976). The clinically healthy gingiva did not correspond to a histologically healthy gingiva containing only a few inflammatory cells, probably because the 3-4 wk of perfect oral hygiene were not sufficient to generate histological health. Furthermore, no chronic inflammation of the gingiva, as characterized by a predominance of plasma cells, was observed after 3 wk without oral hygiene. Thus, more than 3 wk of no oral hygiene are necessary to obtain an established gingival lesion. With increasing gingivitis scores between GI = 0 and GI = 2 there was a significant increase in the percentages of lymphocytes and a significant decrease in the percentages of fibroblasts. With increasing GEFR similar trends in percentages were observed for lymphocytes and fibroblasts. It was concluded that GI scores and GEFR reflect histologic changes in tissue and, hence, are valid indicators of gingivitis development.