Subtherapeutic phenytoin serum levels and loss of seizure control occurred in a 31‐year‐old man due to decreased bioavailability of oral drug. During storage at extreme temperatures, physical changes of the phenytion capsule resulted in altered dissolution characteristics so that only 50% dissolution occurred at 180 minutes compared with 95% in 120 minutes for control capsules. Similar changes were produced in fresh capsules within seven days at high temperature and humidity. The affected patient has metabolic responses to phenytoin that produce marked fluctuations in serum levels with changes in dose. Altered phenytoin serum concentrations may occur with minor dose changes in such patients despite good compliance.