The purpose of this study was to look for a possible explanation for the variation in the incidence rate of melanoma among counties in Washington state. We used data from the Washington State Cancer Registry (WSCR), the Cancer Center at PeaceHealth St. Joseph Hospital in Whatcom County, and the Surveillance, Epidemiology, and End Results registry to get information on melanoma incidence. Demographic and migration records were obtained from the US Census Bureau, the Washington State Department of Licensing (DOL), and the US Internal Revenue Service (IRS). A number of different analytic techniques were used to address our research question, including a multiple regression analysis, time trend comparisons, and an analysis of birthplace data of melanoma patients in Whatcom county. We found a significant association between migration rate from the Southwest (SW) USA and melanoma incidence (P<0.001). Plots of time trend show that melanoma rates track with migration from the SW. In Whatcom county, almost half of all residents were born outside of Washington state, but they accounted for about 70% of all melanoma cases. Our analyses suggest that migration from the SW is an important factor in explaining the variation in melanoma rate among counties in Washington.