To investigate the relationship between keratoacanthoma (KA) and squamous cell carcinoma (SCC), cytokine mRNA in 12 KA and eight SCC were compared. Normal skin was also studied. Reverse transcription polymerase chain reaction (RT-PCR) was used to quantitate mRNA in each sample utilizing DNA standards. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was used as an internal control, and CD3delta as an indication of the T-cell infiltrate. KAs showed a significant increase in interleukin (IL)-10, and a decrease in granulocyte macrophage colony-stimulating factor (GM-CSF) mRNA compared to SCCs. CD3delta mRNA was also increased in the KAs. There was no difference between KAs and SCCs in expression of lymphotoxin-alpha, IL-2, interferon-gamma (IFN-gamma), IL-13, transforming growth factor-beta (TGF-beta), or the pro-inflammatory cytokines IL-8 or tumour necrosis factor-alpha (TNF-alpha). These results indicate that KAs spontaneously resolve in an immunosuppressive environment. KAs grow rapidly over a period of weeks and then involute. It is possible that a suppressed immune response enables unimpeded growth and that the KA cells rapidly undergo the finite number of cell divisions of which they are capable, and then die without reaching immortality.