The International Agency for Research on Cancer (IARC) has classified a number of the chemical constituents reported in cigarette mainstream smoke (MS) as carcinogens. In the international literature, 81 IARC classified carcinogens have been reported historically in MS. Cigarette smoke is a complex aerosol of minute liquid droplets (termed the particulate phase) suspended within a mixture of gases (CO(2), CO, NO(x), etc.) and semi-volatile compounds. The gases and semi-volatiles are termed the vapor phase. Due to early difficulties in inducing carcinomas in laboratory animals following inhalation exposure to MS, the mouse dermal promotion assay became the standard method of comparing the tumorigenic potential of cigarette smoke condensates (the particulate phase of MS nearly devoid of MS gases and having a significant reduction of the semi-volatile components of the vapor phase). Of the 81 IARC carcinogens reported in MS, 48 are found exclusively in the particulate phase, 29 in the vapor phase only, and four IARC carcinogens in both phases. A general comparison of the quantity and potency of the individual carcinogenic constituents of the MS vapor and particulate phases illustrates that the potential carcinogenic contribution from the vapor phase might be significant. Therefore, the mouse dermal promotion assay may not be a sensitive comparator of the tumorigenic potential of different MSs displaying a diversity of vapor phase components. However, when used in a weight-of-the-evidence approach that includes smoke chemistry, in vitro studies using whole smoke and human exposure studies evaluating both vapor and particulate phase smoke constituents, the mouse dermal promotion assay remains an important risk assessment tool as the only test that reproducibly measures the tumorigenic potential of cigarette smoke condensate.