One hundred years ago, a massive explosion occurred in the harbor of Halifax, Nova Scotia, destroying the city and killing more than 2,000 and injuring more than 9,000. It was the worst manmade explosion the world had ever seen, not exceeded until the atomic bomb blast over Hiroshima in 1945. An urgent appeal for assistance came from the survivors, and many volunteers responded. This report describes the prompt and remarkable medical relief effort of the citizens of Massachusetts to help their Canadian neighbors.