Kleefstra syndrome is a rare autosomal dominant genetic disorder caused by haploinsufficiency of the EHMT1 (Euchromatic Histone MethylTransferase 1). Patients with Kleefstra syndrome have following most common symptoms: moderate or severe intellectual deficiency, absence of speech, significant diffuse muscular hypotonia, micro-brachycephaly, congenital defects of heart, kidneys, genitourinary tract and recognizable dysmorphic features of face. The article presents 2 similar clinical cases of Kleefstra syndrome in combination with epilepsy. Both patients, along with a typical clinical picture of the underlying disease, have serial epileptic spasms with an onset after first year of life, modified hypsarrhythmia with tendency to synchronization on the electroencephalogram, pharmacoresistant epilepsy. This indicates that Kleefstra syndrome can include epilepsy as one of symptoms of the disease.