Arterial blood gas (ABG) measurements are used frequently in acute asthma. Because ABGs are expensive and may have significant side effects, a method is needed to identify those patients at risk for a significantly abnormal ABG. We studied the use of peak expiratory flow rates (PEFR) to identify those patients at such risk. Data from 89 emergency visits by 51 asthmatic patients were analyzed. A small but significant correlation between ABG parameters and PEFR was observed (P less than 0.05). No patient with a PEFR greater than or equal to 25% predicted has a PaCO2 greater than 45 mm Hg or pH less than 7.35. This suggests that only those patients with a PEFR less than 25% predicted are at risk for significant hypercarbia or acidosis. We concluded that PEFR may be used as a simple screening tool to safely eliminate ABGs in at least 40% of acute asthmatic patients.