Chronic lymphocytic leukemia (CLL) is the most common type of leukemia occurring in Western nations. In CLL it is well known that the risk of a secondary malignancy is higher than in the normal population. But in Korea, CLL is a rare type of leukemia, so there have been only a few reported cases with a secondary malignancy. CLL is characterized by progressive defects in both cell-mediated and humoral immunity. It is known that defects in the immune system of patients with CLL contribute to the development of a secondary malignancy. We experienced a case of a 71-year-old man who suffered from a chronic cough and was diagnosed with small cell lung cancer coexisting with CLL. Until this case, there was no reported case in Korea of small cell lung cancer coexisting with CLL. We now report a case of small cell lung cancer coexisting with CLL and present a literature review.