We studied p53 protein's pattern of expression, association with retinoid response or resistance, and modulation by retinoid intervention in oral premalignancy. These p53 analyses were included in a prospective trial of the retinoid isotretinoin (1.5 mg/kg/day for 3 months) in 40 patients (45 oral premalignant lesions). Seven nonsmoking subjects (eight oral biopsies) were included as a control. Protein levels of p53 were determined separately for the whole epithelium and the basal, parabasal, and superficial layers. A wide range of accumulated p53 protein levels occurred in 40 (89%) of 45 lesions in basal and parabasal but not superficial layers. No p53 protein was detected in any normal controls. Accumulation of p53 increased in direct association with histological grade (P = 0.0004). An inverse relationship occurred between the levels of accumulated p53 protein and response to isotretinoin (P = 0.006). High-dose isotretinoin did not modulate accumulated p53 protein expression.