Concerning the hypothesis that distinct types of salivary gland cysts may be the starting point of a salivary gland tumour, a histological examination of 1,661 salivary gland cysts was performed in order to analyse the cell types and their proliferative activity. Epithelial alterations were found especially in salivary duct cysts of parotid gland and in mucous retention cysts of minor salivary glands. Characteristic cellular changes were epithelial metaplasias (goblet cells, clear cells, squamous cells) and focal epithelial proliferations with plump or papillary plaques projecting into the cyst lumen. Only in one case had a mucoepidermoid carcinoma developed in the wall of a parotid duct cyst. The epithelial metaplasia and focal proliferative activity in salivary duct cysts is comparable to similar alterations in odontogenic cysts as possible early manifestation of a tumour, especially of an ameloblastoma or mucoepidermoid carcinoma. The differential diagnosis of salivary duct cysts must take primarily cystadenomas and cystic mucoepidermoid carcinomas of well-differentiated type into account.