The aim of the present study was to investigate epithelial cell proliferation in the linings of odontogenic cysts, including three different subtypes of odontogenic keratocyst (OKC), namely simple (non-recurrent), recurrent and basal cell naevus syndrome (BCNS)-associated lesions. Ki67 immunoreactivity in OKC (simple, n = 10; recurrent, n = 8; syndrome, n = 9), dentigerous cysts (DC, n = 5), radicular cysts (RC, n = 5) and normal oral mucosa (n = 7) was studied using a biotin-streptavidin-peroxidase method on paraffin sections after microwave treatment. Ki67+ epithelial cells were counted manually and related to the length of basement membrane (BM) as determined by TV image analysis. Data were analysed by the Mann-Whitney U test. The number of Ki67+ cells in simple OKC linings (53.1 +/- 17.8 cells/mmBM) was similar to that in oral epithelium (42.5 +/- 12.7 cells/mmBM; P > 0.2). However, both contained significantly more Ki67+ cells than DC (3.9 +/- 1.3 cells/mmBM) and RC linings (6.7 +/- 4.8 cells/mmBM; P < 0.006). The epithelial distribution of Ki67+ cells differed between groups, with the percentage of positive cells in basal layers in OKC linings (7.0 +/- 2.1%) being significantly lower than that of oral epithelia (35.9 +/- 5.6%), DC (78.4 +/- 8.4%) and RC (80.6 +/- 7.7%) linings respectively (P < 0.003). Comparison of Ki67 expression within the OKC group revealed no significant difference between simple and recurrent lesions (44.0 +/- 13.8 cells/mmBM; P > 0.3). However, OKC associated with BCNS contained significantly higher numbers of Ki67+ cells (91.8 +/- 35.6 cells/mmBM; P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)