In a longitudinal study between 1991 and 1993 in an endemic area in eastern Sudan, 85 cases of kala-azar (visceral leishmaniasis) were diagnosed, of whom 48 (56%) developed post-kala-azar dermal leishmaniasis (PKDL). Another four cases of PKDL had no clinical history of kala-azar. In children, PKDL was more frequent in the very young; seven of nine kala-azar cases (78%) in the group 0-1 years of age and 13 of 16 (81%) in the group 2-3 years of age developed PKDL. On the average, PKDL occurred 56 days (mean; range 0-180) after the end of treatment of kala-azar. To assess the severity of PKDL, a classification was developed using three grades of severity based on differences in density and distribution of lesions. In young children, PKDL was more severe. Incomplete treatment of kala-azar may be important in the pathogenesis of PKDL. Thirty-one patients were followed-up for at least six months; of these, 20 were not treated (17 healed, two improved, and in one, the condition was unchanged), three healed after incomplete treatment with sodium stibogluconate, and eight were cured after treatment but two required two courses. Considerable morbidity was caused by PKDL and should be taken into consideration in the management and follow-up of kala-azar patients. The high incidence of PKDL may have important implications in transmission.