A study of 34 hospitalized patients with suspected neuroleptic malignant syndrome (NMS) found that 24 had NMS and the other 10 had acute, usually serious, medical problems. There were no demographic, psychopathologic, or treatment-related differences between the groups. NMS patients had more dehydration, cogwheeling, diaphoresis, disorientation, drooling, dysphagia, and rigidity and higher diastolic blood pressure. The groups had similar fevers, heart rates, creatine kinase levels, and white blood cell counts. Three non-NMS patients died during their acute illnesses. Results suggest that considering NMS as a diagnosis and ruling out other acute illnesses such as pneumonia are equally important when a patient on neuroleptic medication becomes medically ill.