Interleukin-5 (IL-5) plays an important role in allergic diseases accompanied by eosinophilia. We examined IL-5 messenger RNA (mRNA) expression in peripheral blood mononuclear cells (PBMC) isolated from 13 patients with bronchial asthma, 1 patient with chronic eosinophilic pneumonia, 2 patients with idiopathic eosinophilia, and 5 control subjects. IL-5 mRNA was expressed in PBMC from 2 of 13 patients with asthma, 1 patient with chronic eosinophilic pneumonia, and 1 of 2 patients with idiopathic eosinophilia. IL-5 mRNA expression was not detected in PBMC from control subjects. PBMC from a patient with chronic eosinophilic pneumonia expressed IL-5 mRNA spontaneously. Prednisolone treatment decreased his clinical symptoms and IL-5 mRNA expression. IL-5 mRNA expression preceded revival of the disease. These observations show that IL-5 plays a role in the condition, accompanied by eosinophilia, and IL-5 mRNa examination in PBMC by means of reverse transcription-polymerase chain reaction may be useful to predict the activity of eosinophilia.