Over a two-year period, 44 patients had an eosinophil count above the 97th centile. Thirteen of these 44 had heart disease presenting within six months of the onset of symptoms. Microfilariasis was the most likely cause of the raised total eosinophil in these 13 patients. In all, the raised eosinophil count we returned to normal by the use of diethylcarbamazine (Banocide). Eleven of the 13 were followed up and eight of them (73%) developed clinical features of cardiac constriction and tricuspid regurgitation. The mean duration of follow-up was two years. Limited cardiac catheterisation studies in six of the patients showed evidence of constriction or of tricuspid regurgitation. The clinical features of these eight patients were indistinguishable from those found in chronic endomyocardial fibrosis. This disease was further suggested by recurrent cerebral embolism in one, and a large pericardial effusion in another; and it was also present in the one patient to come to necropsy. Thus, microfilaria-induced eosinophilia, when high, is frequently associated with heart disease which appears to be chronic endomyocardial fibrosis many months after the eosinophilia has returned to normal.