17 cases of focal epithelial hyperplasia of the oral mucosa (FEH, Heck's disease) were investigated for the presence of human papillomavirus (HPV) nucleic acid sequences by means of in situ DNA hybridization using biotinylated DNA probes of HPV types 1, 6, 11, 13, 16, 18, and 32. Ten of 17 cases were positive for HPV 13 DNA in contrast to 6 of 17 positive cases obtained after application of the HPV 32 probe, with a double infection in one case. The results of our study suggest, that HPV 13 and HPV 32 are very specifically found in lesions of FEH and can be detected in a high percentage of cases using in situ hybridization.