Pallister–Killian syndrome (PKS) is a rare, sporadic, genetic disorder characterized by dysmorphic features, learning disability, and epilepsy. It is caused by a mosaic supernumerary isochromosome 12p (i[12p]). The i(12p) is rarely found in peripheral blood but it is present in skin fibroblasts. Recognition is essential for cytogenetic diagnosis. We describe a male aged 2 years 6 months and a female aged 11 years with PKS and epileptic spasms (ES). This type of seizure is not unusual in patients with brain malformations and with severe developmental delay, but it is sometimes difficult to recognize without video–electroencephalogram studies and could be mistaken for other types of seizure or behavioural manifestations. In these two patients with PKS, spasms had late onset, persisted beyond infancy, and were drug resistant. Clinicians should be aware of this possibility in PKS, which appears to be a rare cause of ES.