OBJECTIVE: To investigate empirically the possibility of an environmental trigger for autism among genetically vulnerable children that is positively associated with precipitation. DESIGN: We used regression analysis to investigate autism prevalence rates and counts first in relation to mean annual county-level precipitation and then to the amount of precipitation a birth cohort was exposed to when younger than 3 years, controlling for time trend, population size, per capita income, and demographic characteristics. In some models, we included county fixed-effects rather than a full set of covariates. SETTING: Counties in California, Oregon, and Washington. PARTICIPANTS: Children born in California, Oregon, and Washington between 1987 and 1999. Main Exposure County-level precipitation. MAIN OUTCOME MEASURES: County-level autism prevalence rates and counts. RESULTS: County-level autism prevalence rates and counts among school-aged children were positively associated with a county's mean annual precipitation. Also, the amount of precipitation a birth cohort was exposed to when younger than 3 years was positively associated with subsequent autism prevalence rates and counts in Oregon counties and California counties with a regional developmental services center. CONCLUSIONS: These results are consistent with the existence of an environmental trigger for autism among genetically vulnerable children that is positively associated with precipitation. Further studies focused on establishing whether such a trigger exists and identifying the specific trigger are warranted.