This paper deals with variations in the relationship between the annual rainfall of any given station and that of one other station, near or far. The variation is stated as a ratio of the normal or average relationship between the two stations. The most probable deviation of this ratio from its mean is a measure of the probable error of estimate in restoring a missing record of annual rainfall by reference to another station when the normal relationship is known. The records of 13 stations on Oahu for a period of 35 years were used and measures of geographic variation determined for similar and near-by pairs of stations as well as for dissimilar and remote pairs. In general, probable geographic variations for any year are found to be as low as 10 per cent for the most closely similar pairs of stations and as high as 45 per cent for the more remote and least similar pairs. Charles K. Wentworth, Principal Geologist, Honolulu Board of Water Supply.