Objective To examine the association between midpregnancy dietary patterns and pregnancy-associated hypertension (PAH).Design A prospective longitudinal cohort study. Setting Denmark.Population About 55 139 Danish women with single enrolments and recorded food frequency questionnaire dates with complete information on dietary intake. Methods Women were eligible if they could speak Danish and were planning to carry to term. Diet was assessed using a validated semi-quantitative 360-item food frequency questionnaire and dietary patterns were derived using factor analysis.Main outcome measures Gestational hypertension (GH) and preeclampsia (PE).Results Disease prevalence was 14% for GH (5491/39 362); 2% for PE (1168/54 778), and 0.4% for severe PE (234/55 086). Seven dietary patterns were characterised in the population, of which two were associated with PAH. The Seafood diet characterised by high consumption of fish and vegetables was inversely associated with the odds of developing GH [odds ratio (OR) 0.86; 95% CI 0.77-0.95)] and PE (OR 0.79; 95% CI 0.65-0.97). The Western diet characterised by high consumption of potatoes (including French fries), mixed meat, margarine and white bread increased the odds of developing GH (OR 1.18; 95% CI 1.05-1.33) and PE (OR 1.40; 95% CI 1.11-1.76). No association was seen with severe PE. ConclusionsWe found protective associations of Seafood diet and harmful associations of Western diet with PAH. Dietary interventions encouraging the reduction of Western diet may contribute to a decrease of PAH.Tweetable abstract Western diet increases (Seafood diet decreases) the likelihood of developing pre-eclampsia among Danish pregnant women.