Small cell lung carcinoma (SCLC) usually grows in a pure form with no other associated histological components. However, combined small cell lung carcinoma (cSCLC), which is accompanied by other histological components (cSCLCs) may sometimes occur. Herein, we analyzed the tumorigenesis of cSCLC containing a demarcated area of pure SCLC. A 76-year-old man had a 25-mm mass in the perihilar portion of his right upper lung. Histologically, the cSCLC contained two relatively demarcated areas: one area composed of pure SCLC cells and another area of SCLC, squamous-like component (SLC), and spindle cell carcinoma (SpCC) cells. Loss of heterozygosity (LOH) was observed at allele 3p in all tumor components and at 22q in the pure SCLC component. Histological and immunohistochemical analysis and LOH study suggested that all components were likely to be monoclonal in origin and revealed that the pure SCLC component was not the precursor of the cSCLC. In the tumorigenesis of this case, the pure SCLC and the cSCLC may have originated from a common pluripotent tumor cell and then diverged, although we cannot state this conclusively. Further studies with more cases are necessary to test this theory.