To analyze the acid-base abnormalities in patients with status epilepticus. We retrospectively reviewed the acid-base disturbances in 38 consecutive patients who had been admitted to the emergency department at a Mayo-affiliated hospital because of status epilepticus between 1982 and 1993. On the basis of results of arterial blood gas analyses, the acid-base disorders were categorized. In addition, chest roentgenograms and electrocardiograms were reviewed for pulmonary infiltrates and cardiac arrhythmias. Arterial blood gas analysis performed immediately after admission revealed an acid-base abnormality in 32 of 38 patients (84%). Respiratory acidosis was most common (N = 16; 42%) and occurred either alone or in combination with metabolic acidosis. Pulmonary infiltrates were not more common in patients with respiratory acidosis than in other patients. Although cardiac arrhythmias were more common in patients with respiratory or metabolic acidosis (42%) than in those with respiratory alkalosis or normal blood gas values (36%), this difference was not statistically significant. We conclude that respiratory acidosis with or without metabolic acidosis is common in patients who have status epilepticus. Respiratory acidosis was not predictive of the final outcome. In our study patients, all acid-base abnormalities resolved spontaneously.