Dravet syndrome, often caused by mutations of SCN1A-gene, presents with prolonged clonic, generalized or unilateral seizures often occurring with fever during the first year of life, followed by usually severe epilepsy. The EEG, normal at the outset, later shows generalized and focal epileptic activities. The psychomotor development deteriorates, but little is known about the time course of the cognitive impairment and its relationship with seizures severity. We describe here the progressive neurocognitive decline in two children (one male), carrying de novo SCN1A truncating mutations and presenting with different epileptic phenotypes. The children were longitudinally assessed from the ages of 11 and 23 months until the age of 7 and 8 years, using the same scales to measure the developmental competence in various domains. Both had seizures during the first year of life, unilateral clonic in one and myoclonic in the other, but the subsequent epilepsy severity and the characteristics of the EEG diverged. One child had drug-resistant but rare generalized seizures and isolated EEG spike-wave paroxysms, while the other developed extremely frequent clusters of polymorphic seizures and generalized plus multifocal EEG epileptic activities. MRI was normal in both. A clear developmental delay begun before the age of 2 years in both children and the cognitive profile continued to worse, with some differences between different domains, irrespectively to the different course of their epileptic histories. Our observations are consistent with the hypothesis that SCN1A-mutations can be responsible not only for epilepsy, but also for early and progressive severe mental impairment.