Mechanical ventilation (MV) during exacerbation of asthma or chronic obstructive pulmonary disease (COPD) is unequivocally needed when apnoea, cardiorespiratory arrest, coma, hypoxia or treatment failure is present. The need is less clear when the patient can respond, has intact airway reflexes and spontaneous respiration. In this situation, acidosis is an important factor in the decision to institute MV. This study aimed to provide a clinical means of identifying patients with acute respiratory acidosis (ARA) in a setting where blood gas analysis is unavailable. We undertook a prospective, observational study of consecutive patients who presented to two emergency departments with severe and life-threatening exacerbation of asthma or COPD. Each underwent clinical assessment, treatment and blood gas analysis. The outcome measure was ARA or mixed ARA and metabolic acidosis. A total of 127 episodes in patients aged 15-90 years (65.3% males and 34.7% females) were included in the study. Of these, 62.2% had asthma and 37.8% had COPD; 71.7% had life-threatening and 28.3% had severe attacks. Overall, the adjusted odds ratio (and 95% confidence intervals) for predictors of ARA were 7.09 (1.79-28.06) for drowsiness, 4.11 (1.31-12.88) for flushing, 3.34 (1.01-11.02) for having COPD and 2.86 (1.01-8.07) for intercostal retractions. In conclusion, with drowsiness, the likelihood of ARA is about seven times higher. The presence of flushing, COPD and intercostal retractions also increase the risk of ARA.