Precipitation estimates from various sources for the different Great Lakes drainage basins are reviewed. To check the comparative accuracy of the estimates, they are correlated with the net basin supply (runoff) values for each basin. The best correspondence between net basin supply and precipitation is indicated for the smaller basins—Erie and Ontario—and the poorest for the larger basins, Michigan-Huron and Superior. It appears that reasons for the poorer relationship in the case of the larger basins include the use of calendar-year, rather than water-year, net basin supply and precipitation data, and the use of a varying number instead of a fixed-number of stations. The use of a fixed number of stations appears to climinate the possibility of the inclusion of time trend errors.