Subtypes of status epilepticus (SE) without tonic-clonic convulsions (nontonic-clonic SE) present as altered consciousness sometimes with subtle motor activity and are important to consider in the differential diagnosis of patients with unexplained altered consciousness. Other patients may have altered consciousness with intermittent ictal activity on electroencephalography (EEG) that represents probable SE, but have other medical conditions that may be contributing to altered consciousness. EEG is the only reliable way to make the diagnosis of nontonic-clonic SE and we make emergency EEG available on a 24-h basis at our hospital. To determine how often definite or probable nontonic-clonic SE was detected by EEG we prospectively collected data on all cases where physicians ordered EEG to evaluate altered consciousness or possible SE. Out of 198 cases with altered consciousness but no clinical convulsions, 74 (37%) showed EEG and clinical evidence of definite or probable nontonic-clonic SE. Forty-two episodes (57%) were probable or definite complex partial SE, 29 (39%) were probable or definite subtle generalized SE, and three (4%) were myoclonic SE. In 23 SE cases altered consciousness was the only clinical sign at the time of diagnosis; subtle motor activity was present in 36 others. Neither clinical signs nor prior history predicted which patients showed SE on EEG. Nontonic-clonic SE followed a cerebral infarction in 16 cases. Contrary to other reports, we found no relationship between duration of SE and EEG pattern. Subtle generalized SE occurred most commonly in the setting of a diffuse brain injury rather than evolving from convulsive SE. This study demonstrates that nontonic-clonic SE is a common finding in patients with unexplained altered consciousness and EEG is necessary in the evaluation of these patients.