PURPOSE: To evaluate the changes of differential counts and lymphocyte subsets in cancer patients' leukocyte before and after radiotherapy. METHODS AND MATERIALS: From Dec. 1994 to May 1995, the changes of leukocyte and its subsets in 16 patients who received radiotherapy in the Dept. of Radiation Oncology of Dong-A University Hospiatal were investigated. Radiation was delivered from 2700 cGy to 6660 cGy with median dose of 5400 cGy. The results of pre- and post-radiotherapy were analyzed by paired T-test. The results of patients who received < 50 Gy and > or = 50 Gy were analyzed by wilcoxon test. RESULTS: Before and after radiotherapy, there was not any significant differences in the counts of leukocyte, granulocyte and monocyte. A remarkable decrease was noted in lymphocyte counts after radiotherapy(p=0.015). T cells, B cells and natural killer cells were also decreased in number after radiotherapy but it was not significant statistically. T helper cells and T suppressor cells were also decreased in number(p>0.05). The ratio of T helper/suppressor cell was decreased from 1.52 to 1.11 and it was significant statistically(p=0.016). The portion of T suppressor cell among all T cells was increased after radiotherapy (p=0.0195). No significant difference was observed in the analysis of leukocyte and its subsets between patients who reveived < 50 Gy and > or = 50 Gy. CONCLUSION: Radiotherapy caused remarkable decrease in lymphocyte count and its subsets. Among all lymphocyte subsets, T helper cell might be the most vulnerable to radiation, considering decreased ratio of T helper/surppressor cell count after radiotherapy.