Purpose: Mutations of the alpha-1 subunit sodium channel gene (SCN1A) cause severe myoclonic epilepsy of infancy (SMEI). To date, over 300 mutations related to SMEI have been described. In the present study, we report new SCN1A mutations and the clinical features of SMEI cases. Materials and Methods: We studied the clinical and genetic features of nine patients diagnosed with SMEI at the Pediatric Neurology Department of Istanbul Medical Faculty. Results: Five patients had nonsense mutations, two had missense mutations, one had a splice site mutation and one had a deletion mutation of the SCN1A gene. Mutations at c.3705+5G splice site, p.trip153X nonsense mutation and deletion at c.2416_2946 have not been previously described. The seizures started following whole cell pertussis vaccination in all patients. The seizures ceased in one patient and continued in the other eight patients. Developmental regression was severe in three patients, with frequent status epilepticus. The type of mutation was not predictive for the severity of the disease. Two of the three patients with severe regression had nonsense and missense mutations. Conclusions: Dravet syndrome can be result of several different types of mutation in SCN1A gene. Onset of the seizures after pertussis vaccination is an important clue for the diagnosis and neuro-developmental delay should be expected in all patients.