BACKGROUND In multiple lung cancers (MLCs), distinction between intrapulmonary metastases and multiple primary tumors is important for staging and prognosis. In this study, we have investigated histopathologic prognostic factors of patients with MLCs. METHODS Histologic subtype, size differences, lobar location, lymphovascular invasion (LVI), size of the largest tumor, nodal status, number of tumors, morphology of tumor periphery, and immunohistochemical profiles using eight antibodies, were analyzed in 65 patients with MLCs. RESULTS There was no significant difference in the survivals of patients with multiple primary tumors and intrapulmonary metastases, as determined by the Martini-Melamed criteria (p=0.654). Risk grouping by four histologic parameters, LVI, margin morphology, size differences, and lobar locations of paired tumors were prognostic. The patients with one or two of aforementioned parameters had significantly longer survival than those with three or four parameters (p=0.017). In patients with largest mass (< or =5 cm), the risk grouping was found to be an independent prognostic factor (p=0.022). However, differences in immunohistochemical staining were not related to patients' survival. CONCLUSIONS A risk grouping of MLC patients by using combinations of histologic parameters can be a useful tool in evaluating the survival of patients with MLCs, and may indicate clonal relationship between multiple tumors.