ObjectiveTo determine the applicability of the Acute Physiology and Chronic Health Evaluation (APACHE) II scoring system in predicting outcome in a subgroup of critically ill obstetrical patients.DesignRetrospective data collection.SettingA multidisciplinary intensive care unit (ICU) in a university hospital.PatientsAll patients (n = 1,670) admitted for >24 hrs to the ICU during an 8-yr period, of whom 58 were obstetrical patients and 120 were nonobstetrical young women.Measurements and Main ResultsThe mean APACHE II score in the obstetrical group was 11, with a mortality risk of 16.6%. In this group, the mortality ratio, which is the ratio between actual and predicted mortality rate, was low (0.416) and significantly (p= .021) different from the expected mortality ratio of 1. The mean APACHE II score in the group of nonobstetrical young women was 10, with a mortality risk of 10.17%. In all nonobstetrical ICU patients including all the admitted patients excluding the obstetrical patients, the mean APACHE II score was 15, with a mortality risk of 24.18%. The mortality ratio in the nonobstetrical young women group and in the nonobstetrical ICU patient group was 0.986 and 1.006, respectively, which was nonsignificantly different from the expected mortality ratio.ConclusionsObstetrical patients requiring intensive care in our ICU had a better outcome than predicted, as expressed by a low mortality ratio. Various explanations that may be applicable to any subgroup of critically ill patients with a different mortality ratio are presented. The subgroup itself may be uniquely different, similar to our obstetrical patients with their physiologic changes of pregnancy. Another explanation may relate to an improvement in care of the subgroup and therefore a better outcome. (Crit Care Med 1994; 22:1412–1414)