During 1991, tropical cyclones formed within seven different tropical cyclone basins, located within various parts of the Atlantic, Pacific and Indian Oceans. During the year, a total of 100 systems formed with 75 of these developing further and were named by the responsible warning centre. The strongest tropical cyclone of the year was Typhoon Yuri, which was estimated to have a minimum barometric pressure of 895 hPa (26.43 inHg). The deadliest tropical cyclone was Cyclone BOB 01, which caused 138,866 fatalities in Bangladesh, Northeastern India, Myanmar, Yunnan, while the costliest was Typhoon Mireille, which caused an estimated $10 billion USD in damage after striking Japan. Four Category 5 tropical cyclones formed in 1991.