The effects of introducing Modified Early Warning scores to identify medical patients at risk of catastrophic deterioration have not been examined. We prospectively studied 1695 acute medical admissions. All patients were scored in the admissions unit. Patients with a Modified Early Warning score > 4 were referred for urgent medical and critical care outreach team review. Data was compared with an observational study performed in the same unit during the proceeding year. There was no change in mortality of patients with low, intermediate or high Modified Early Warning scores. Rates of cardio-pulmonary arrest, intensive care unit or high dependency unit admission were similar. Data analysis confirmed respiratory rate as the best discriminator in identifying high-risk patient groups. The therapeutic interventions performed in response to abnormal scores were not assessed. We are convinced that the Modified Early Warning score is a suitable scoring tool to identify patients at risk. However, outcomes in medical emergency admissions are influenced by a multitude of factors and so it may be difficult to demonstrate the score's benefit without further standardizing the response to abnormal values.