The elderly have a higher mortality rate during heat waves and may, therefore, have higher morbidity. We investigated the effects of high ambient environmental temperature on emergency department (ED) function and patient (age >64 years) morbidity. A retrospective case-control study of ED presentations at an Australian metropolitan hospital from September 2005 to May 2010 was undertaken. Cases comprised 1297 ED presentations surrounding heat threshold days. These were compared with randomly selected presentations on nonthreshold days (3 controls: 1 case), analyzing patient morbidity and ED function variables: triage category, presenting complaint, demographics, arrival mode, time to doctor, ED length of stay (LOS), ED disposition, and discharge diagnoses. A greater proportion of cases presented with "weakness," and were diagnosed with "dehydration." There was little effect on ED function, with ED LOS 24 minutes longer for cases and time to doctor 3 minutes shorter. This study found limited evidence of increased morbidity in the elderly during heat wave conditions.