The large run-up heights observed on the lee side of a circular island are investigated, using the shallow-water equations, to understand the devastating run-up heights reported in the lees of both Babi and Okushiri Islands during the Flores 1992 and the Hokkaido 1993 tsunamis. Specifically, the effects of the crest length of incident waves are examined both numerically and experimentally. It is found that the run-up heights around a circular island are strongly dependent on the crest length of incident waves when the crest length is less than the base diameter of the island but are almost insensitive if the crest length is greater than twice the base diameter. This observation implies that it may be possible to calculate a worst case scenario of run-up on an island from nearshore events. ¿ 1999 American Geophysical Union