Washington Initiative 1183 (I-1183), a 2012 law that privatized liquor retail sales and distribution in Washington State, USA, has had two opposing effects on liquor purchases: it has increased access to liquor and imposed new fees on retailers and distributors. This study aimed to estimate the effect of I-1183 on monthly alcohol purchases during the post-I-1183 period (June 2012-December 2014) compared with the pre-I-1183 period (January 2010-May 2012).DIFFERENCES-IN-DIFFERENCES STUDY: Setting and participants The study included households participating in the Nielsen Consumer Panel Dataset living in metropolitan and surrounding areas in Washington State and 10 control states. Measurements Outcomes were alcohol purchases by type (ounces of liquor, wine, beer and total alcohol or ethanol). Findings I-1183 was associated with a 6.34-ounce (P < 0.001) and a 2.01-ounce (P < 0.001) increase in monthly liquor and ethanol purchases, respectively, per household in the post-policy period spanning 31 months compared with monthly purchases in control states. In a longitudinal subgroup analysis, low and moderate alcohol purchasers increased monthly purchases of ethanol and high purchasers decreased purchases of ethanol. Conclusions Enacting 'Washington Initiative 1183', a law that privatized sale and distribution of liquor and imposed new fees on retailers and distributors, appears to have been associated with an approximate 82% increase in monthly liquor purchases and 26% increase in monthly ethanol purchases by households in metropolitan and surrounding areas in Washington State, USA.