The prevalence of oral mucosal lesions in Sweden is virtually unknown. Available prevalence figures concern but a few types of lesions and relate to highly selected, special populations. Investigations on general populations are sparse also in other countries. The aim of the present study was to survey the occurrence of oral mucosal lesions in a relatively large general population in Sweden. The study was performed in collaboration with a health screening organization in the middle of Sweden. The populations of two municipalities in the County of Uppsala were investigated. A mobile health screening group visited several places in the selected area and during the time of the present investigation a total of 30,118 persons, aged 15 years or above, were summoned. Of these, 62% or 18.659 individuals attended and were examined. The non-participants became the subject of a special investigation for which every fifth individual, or 2.292 persons, was randomly sampled. After a second summons, 933 persons presented for examination. Through personal contact, an additional 741 individuals were examined in private homes, places of work, hospitals, old age homes and other institutions. The final non-participation, representing 10.3% of the total population, was considered to have but a marginal influence on the results of the study. The diagnostic procedure was based on clinical criteria set forth especially for the investigation. The validity of the final diagnoses was evaluated by, for instance, inter-examiner tests and comparisons between clinical diagnoses and histologic descriptions and was found to be acceptable. Tests of the reliability were, among else, undertaken through re-examinations and indicated, that underregistration apparently occurred for a few of the lesions investigated. The prevalences of about 60 oral mucosal lesions were recorded and compared with previous findings. Notably high prevalences were found for focal epithelial hyperplasia (0.11%), leukoedema (49.07%), geographic tongue (8.45%) and lichen planus (1.85%). For some lesions prevalence figures are difficult to compare with findings from previous studies. This includes lesions which are directly or indirectly related to local etiologic factors such as denture status and tobacco habits. As regards these factors basic information has been collected and will be used for further analyses. The collected total material has also been designed to form the basis for longitudinal studies of, for example, precancerous lesions.