Clinical features and electroencephalographic findings of two patients affected by a previously unreported cyclin-dependent kinase-like 5 (CDKL5) gene mutation are described. Both patients had the Hanefeld variant phenotype with early-onset seizures, but different degrees of clinical severity. In fact, patient 1 was not drug-resistant and is responding to a single drug. On the contrary, patient 2, like most reported cases, has severe epilepsy, exhibits electroencephalographic changes, and is drug resistant. We suggest that the pseudoperiodic patterns observed on the EEGs for these cases represent this genetic form of epilepsy, though differing in frequency, voltage, and associated patterns. This is in agreement with data reported by other authors indicating that no unique pattern can be identified in subjects with CDKL5 mutations. Thus, a CDKL5 investigation should be performed in developmentally delayed patients with early-onset seizures, including drug-resistant subjects with severe EEG changes, as well as in patients with milder, drug-responsive forms of epilepsy.