To determine the association between assisted reproductive technology (ART) treatment and the rate of combined gestational hypertension (GH), preeclampsia (PE).Retrospective population study.Not applicable.A total of 596,520 mothers (3.6% ART mothers) who gave birth between 2007 and 2011.Not applicable.Comparison of the rate of GH/PE for ART and non-ART mothers, with odds ratio (OR), adjusted odds ratio (AOR), and 95% confidence interval (CI) used to assess the association between ART and GH/PE.The overall rate of GH/PE was 4.3%, with 6.4% for ART mothers and 4.3% for non-ART mothers. The rate of GH/PE was higher for mothers of twins than singletons (12.4% vs. 5.7% for ART mothers; 8.6% vs. 4.2% for non-ART mothers). The ART mothers had a 17% increased odds of GH/PE compared with the non-ART mothers (AOR 1.17; 95% CI, 1.10-1.24). After stratification by plurality, the difference in GH/PE rates between ART and non-ART mothers was not statistically significant, with AOR 1.05 (95% CI, 0.98-1.12) for mothers of singletons and AOR 1.10 (95% CI, 0.94-1.30) for mothers of twins.The changes in AOR after stratification indicated that multiple pregnancies after ART are the single most likely explanation for the increased rate of GH/PE among ART mothers. The lower rate of GH/PE among mothers of singletons compared with mothers of twins suggests that a policy to minimize multiple pregnancies after ART may reduce the excess risk of GH/PE due to ART treatment.