Serious dermatologic adverse events such as erythema multiforme (EM) and Stevens-Johnson syndrome/toxic epidermal necrolysis (SJS/TEN) have been reported in patients receiving antiepileptic drugs (AEDs) and cranial radiotherapy (RT). Given the frequency of AED-associated rashes and the infrequency of serious dermatologic adverse events after cranial RT, we sought to further assess the prevalence of cutaneous eruptions in patients receiving an AED before and after cranial RT. We reviewed medical records of patients taking AEDs while undergoing RT for a high-grade glioma and recorded demographic, disease, and treatment parameters, as well as the development of rashes. Rashes were found in 19 % of patients taking AEDs. Phenytoin was most commonly implicated (93 %) in rash formation compared with other AEDs (P < 0.0001), both before and during RT. Most rashes (76 %) occurred before starting RT (P < 0.0001). However, of those during RT, most were associated with phenytoin compared with other AEDs (P = 0.002). One case of SJS was noted in a patient receiving phenytoin prior to RT. While rashes were slightly less prevalent in patients receiving temozolomide compared with those not receiving temozolomide (3.4 vs 4.8 %), this difference was not statistically significant (P = 0.65). Rashes are relatively common in patients receiving AEDs, with the highest incidence associated with phenytoin. However, the risk of serious dermatologic events is low. There did not appear to be an association between the receipt of cranial radiotherapy and the development of AED-associated rash with phenytoin or other AEDs.