One hundred and forty-eight (72%) of 205 allogenic bone marrow transplant recipients developed mucosal lesions in the oral cavity during the aplastic period after transplantation. Lesions were most frequent 8 to 9 days after transplantation. Factors that correlated with the development of mucosal lesions after univariate and multivariate analysis were a marrow cell dose of less than 3 x 10(8) cells/kg body weight, (p less than 0.0001), prolonged aplastic period (white blood cell count less than 0.2 x 10(9) cell/l during more than 14 days (p less than 0.005), herpes simplex virus-1 seropositivity of the recipient (p less than 0.01), and conditioning with total body irradiation (p less than 0.02).