The objective of this controlled pilot study was to determine if mRNA coding for interleukin-5 (IL-5), a cytokine that promotes eosinophil differentiation, growth, and migration, could be detected in three T-cell lymphomas that were infiltrated extensively by eosinophils. To detect mRNA coding for IL-5, we performed an RNA polymerase chain reaction on mRNA extracted from three T-cell lymphomas with eosinophilia and from 29 positive and negative validation controls. Using this procedure, we detected a 293-base pair, IL-5-specific amplification product in the three cases of T-cell lymphoma with eosinophilia and in 11 of 12 positive validation controls, including 10 cases of Hodgkin's disease with eosinophilia. IL-5 mRNA was not detectable in the 17 negative validation controls. This preliminary study suggests that IL-5 mRNA is detectable by polymerase chain reaction in three cases of T-cell lymphoma with eosinophilia.