Analysis of 13,430 arterial blood samples demonstrated that metabolic alkalemia was the most common acid-base disturbance encountered in the hospital setting, being present in 51% of patients with abnormal acid-base status. Respiratory alkalemia was encountered in 29%, respiratory acidemia in 27%, and metabolic acidemia in only 12%. Evaluation of those blood gases with metabolic alkalemia, using 95% confidence-limit bands for acid-base disturbances, showed that the metabolic alkalemia was pure in 70% of the cases, mixed with respiratory acidemia in 18%, and combined with respiratory alkalemia in 12%. The adverse effects of alkalemia may be subtle but often significant. Alveolar hypoventilation in response to metabolic alkalemia, without evidence of primary respiratory disease, was more frequent than is generally suspected. This hypoventilation often directly results in hypoxemia, as well as atelectasis with worsened ventilation/perfusion match-up. Alkalemia shifts the oxyhemoglobin dissociation curve to the left, increasing hemoglobin's affinity for oxygen, thus, limiting oxygen release at the tissue level. The presence of metabolic alkalemia makes it more difficult to wean patients from assisted ventilation. A clearer understanding of the frequency and significance of metabolic alkalemia should lead to more appropriate therapy to prevent or correct this acid-base disturbance and should result in a decreased morbidity and mortality in critically ill patients.