Dravet syndrome is a severe infantile onset epileptic encephalopathy associated with mutations in the sodium channel alpha 1 subunit gene SCN1A. To date no large studies have systematically examined the prognostic, clinical and demographic features of the disease. We prospectively collected data on a UK cohort of individuals with Dravet syndrome during a 5-year study period and analysed demographic information based on UK population and birth figures. From structured referral data we examined a range of clinical characteristics including epilepsy phenotype, seizure precipitants, electroencephalography data, imaging studies, mutation class and response to medication. Predictors of developmental outcome were determined by logistic regression. We identified 241 cases with SCN1A mutation-positive Dravet syndrome, 207 of which were UK-based. The incidence of mutation-positive Dravet syndrome is at least 1:40 900 UK births. Clinical features predicting a worse developmental outcome included status epilepticus (odds ratio = 3.1; confidence interval = 1.5-6.3; P = 0.003), interictal electroencephalography abnormalities in the first year of life (odds ratio = 5.7; confidence interval = 1.9-16.8; P = 0.002) and motor disorder (odds ratio = 3.3; confidence interval = 1.7-6.4; P < 0.001). No significant effect was seen for seizure precipitants, magnetic resonance imaging abnormalities or mutation class (truncating versus missense). Abnormal magnetic resonance imaging was documented in 11% of cases, principally with findings of non-specific brain atrophy or hippocampal changes. Sodium valproate, benzodiazepines and topiramate were reported as being the most helpful medications at the time of referral. Aggravation of seizures was reported for carbamazepine and lamotrigine. The identification of factors influencing prognosis both aids counselling and encourages early, syndrome-specific therapy. Prevention of status epilepticus with regular medication and emergency protocols is important and may influence developmental outcome.