Objective: The aim of this study was to investigate whether the occurrence of adulthood acute myelogenous leukemia (AML) and acute lymphocytic leukemia (ALL) had increased before and after the Chernobyl accident in the Hospital of the Hacettepe University Faculty of Medicine, one of the major tertiary care referral centers in Turkey. In addition, any increase in the number of patients with leukemia presenting from the Black Sea Region (BSR; the northern part of Turkey), which was presumed to be the most influenced region by ionizing radiation before and after the disaster was also assessed. Material and Methods: The study consisted of patients diagnosed with AML and ALL and hospitalized between 1976 and 1994 years. This interval was divided into two periods according to the date of the Chernobyl accident. The period before the Chernobyl accident (BCP) covered between 1976 and 1985 and the period after the Chernobyl accident (ACP) covered between 1989 and 1994. The hospital registration forms were retrospectively analyzed to determine the number of patients with AML and ALL in those periods. In addition, patient files were reviewed to record the place of residency, which was not included in the registration records. Results: The numbers of patients with AML and ALL in BCP and ACP were 194 and 380, and 96 and 151, respectively. There was a linear upward trend for each disease in BCP. These trends disappeared for each disease in ACP. The distribution of patients with AML and ALL in BCP was significantly different when compared to the distribution in ACP (p<0.00 and p=0.005, respectively). After eliminating the trends by difference transformations, the differences between the periods disappeared for each disease (p=0.92 and p=0.60, respectively). The number of AML and ALL patients presenting from the BSR did not show us on: T e disappe- a significant difference between the BCP and ACP (p= 0.74 and p= 0.36, respectively). Conclusion: The disapperance of the significant differences between periods after the elimination of the trends in BCP for each disease indicate that the differences were due to the significant linear increases in the numbers of patients with each disease in BCP and not to periodical changes. In addition, the number of patients presenting from the BSR after the Chernobyl disaster was not significantly different.