His bundle electrocardiograms were recorded from a patient with previously unexplained PR prolongations or shortenings suggestive of both type I and II second degree AV block. The conduction disturbances were due to the association of concealed His bundle depolarizations (H') not propagated to atria or ventricles with first degree AV block in the His bundle. These data strongly suggest that frequent spontaneous His bundle depolarizations are another manifestation of a disease process involving the His bundle, a kind of "Sick--His bundle syndrome".