In 47 of 227 dental students intraoral vesicles developed after multiple alginate impressions. The lesions were generally solitary and clear, and appeared within 24 to 48 hours after the impression. They were most frequently located inside the vermilion border of the lips and resolved spontaneously in 2 to 5 days. The purpose of this study was to determine the cause of these reactions. Histopathologically one lesion was suggestive of a contact allergy. Cutaneous patch tests, which proved negative, were performed on 14 students to determine whether an allergy to the alginate flavoring existed. The surface of three lesions were cultured and the organisms identified. Contamination studies were carried out on seven unopened containers of the alginate powder and resulted in the isolation of some organisms similar to the mucosal cultures; however, no relationship can be proved. These findings indicate that the cause of the vesicles remains unknown, and further studies are necessary to establish the cause.