Oral epithelial dysplasia, the histopathologic marker of a premalignant disorder of the mouth mucosa, may present clinically as leukoplakia, erythroplakia, or leukoerythroplakia. Its presence in lesions of the oral mucosa is predictive by a variable rate (6.6% to 36%) of transformation to invasive squamous cell carcinoma. We studied the clinical features and microscopic slides of 308 case of oral epithelial dysplasia in our biopsy service and retrospectively evaluated 44 of these with follow-up data for transformation to invasive squamous cell carcinoma. Forty-four patients had follow-up of more than 6 months with a mean follow-up time of 18.4 months. Twenty (45%) were clinically free of disease and 15 (34%) had recurrence of the dysplasia. An additional two cases of lower grades of the disease recurred as carcinoma in situ. Invasive squamous cell carcinoma developed in seven (16%) in a mean transformation time of 33.6 months.