OBJECTIVE: To estimate the maternal genetic contribution to the hypertensive diseases of pregnancy. DESIGN: A cohort study of female twins with information on hypertensive diseases of pregnancy obtained by questionnaire screening, and verification of diagnosis from hospital or general practitioner records. SETTING: A volunteer twin registry in the UK with recruitment through the media without reference to pregnancies or disease status. POPULATION: Adult female, same-sex twin pairs who completed a pregnancy history questionnaire and consented to record inspection. MAIN OUTCOME MEASURE: Self-reported and hospital-validated diagnosis of non-proteinuric pregnancy hypertension and of pregnancy hypertension with proteinuria (pre-eclampsia). RESULTS: Self-reported pre-eclampsia had a heritability of 0.221 and non-proteinuric hypertension of 0. 198. However, none of the pairs who were self-reported as concordant for pre-eclampsia were confirmed from hospital records. Using hospital records, the heritability of pre-eclampsia was 0 and 0.375 for non-proteinuric hypertension. Using a model treating pre-eclampsia as a separate disease from non-proteinuric hypertension, and assuming that the next pair identified was both monozygotic and concordant for pre-eclampsia, the estimated heritability of pre-eclampsia remained 0 (95% CI 0-0.49). Using a threshold model in which non-proteinuric hypertension is treated as a mild form of pre-eclampsia, heritability is estimated at 0.247 (95% CI 0.23-0.454). CONCLUSION: Neither non-proteinuric hypertension nor pre-eclampsia are inherited in simple Mendelian fashion. The genetic contribution to multi-factorial inheritance is smaller than hitherto believed.