This letter is a comment on the somewhat contradictory reports of Shone, et al. and Gersony, et al. concerning the pathogenesis of endocardial fibroelastosis. The former suggests a relationship between "positivity" of mumps skin tests and the clinical diagnosis of E.F.E. and the latter denies it. Both suggest that cross-reactivity with other myxoviruses must be considered. Also, Gotoff in commenting on a similar presentation by St. Geme, et al. asked if skin tests with parainfluenza viruses had been carried out.