The difference in death rate among medical and surgical inpatients having alkalemia is incompletely defined. This prospective study was done to identify the incidence, mortality, and clinical factors that resulted in a poor outcome in both groups of patients. We screened a total of 10,811 consecutive arterial blood gas values; in 4,427 (40.9%) the pH was greater than 7.44, and in 2,577 (23.8%) the pH was greater than 7.48. The study group consisted of 409 patients (213 medical, 196 surgical) whose pH value was greater than 7.48; 253 of these patients (61.9%) had pure respiratory alkalosis, 120 (29.3%) had mixed respiratory and metabolic alkalosis, 27 (6.6%) had a mixed acid base disorder, and nine (2.2%) had pure metabolic alkalosis. Overall group mortality was 27.9% and increased as pH values rose, reaching 48.5% when the pH was greater than 7.60. The overall mortality among medical patients (36.6%) exceeded that among the surgical patients (12.4%) (P less than .05), an observation that applied over a wide range of pH values. Patients having mixed respiratory and metabolic alkalosis had a significantly poorer outcome (44.2% mortality) than the alkalemic group as a whole (27.9% mortality) (P less than .05). Alkalemia-associated illnesses are common in hospitalized patients and are associated with high mortality in both medical and surgical patients, though the death rate is higher among medical patients. Mixed respiratory and metabolic alkalosis appears to be associated with a particularly poor prognosis.