To investigate the possible causal association between squamous-cell carcinomas of the oral cavity and use of toombak, we retrospectively compared the history of use of this substance in 375 patients with squamous-cell carcinomas of the lip, buccal cavity and floor of mouth, and 271 patients with squamous carcinomas of the tongue, palate and maxillary sinus, with 204 patients exhibiting non-squamous oral and non-oral malignant neoplasms and 2,820 individuals who had no malignancy, selected from the general population. The study revealed that the high prevalence of oral cancer in the Sudan is largely due to chronic use of toombak. The adjusted ORs associated with toombak dipping for the first case group, cancer of the lip, buccal cavity and floor of mouth in comparison with the hospital and population control groups, were 7.3 and 3.9 (95% confidence limits, 4.3-12.4 and 2.9-5.3) respectively and among long-term users the adjusted ORs were 11.0 and 4.3 (95% confidence limits, 4.8-25.1 and 2.9-6.3) respectively. The elevated risk found when investigating intra-oral cancers of sites in direct contact with toombak quid compared to those with little or no contact, confirms the hypothesis that direct contact with tissues is an important factor in tobacco carcinogenesis in the mouth. The increased risk associated with the use of toombak is of particular concern in view of its wide consumption in the Sudan.