In patients with chronic airflow limitation (CAL), the detection of upper airway obstruction (UAO) by analysis of forced flows can be difficult due to the masking of conventional UAO indices. We analyzed five indices: maximum inspiratory flow at 50% of forced vital capacity (FIF50), the ratio of maximum expiratory to inspiratory flow at 50% of forced vital capacity (FEF50/FIF50), the ratio FEV1/PEFR, the ratio FEV1 to forced expiratory volume in 0.5 s (FEV1/FEV0.5), and the ratio maximum voluntary ventilation (MVV)/FEV1, to determine their usefulness in evaluating patients with simultaneous UAO and CAL. One hundred and thirty-seven patients participated: 54 had UAO alone, 23 presented simultaneous UAO and CAL and 60 suffered from CAL with no evidence of UAO. The patients with UAO and CAL on the average presented fewer abnormal indices and these were less severely altered. Twenty-seven of the 60 with CAL alone presented at least one abnormal index, but in no case were more than two present. FEF50/FIF50 and FEV1/PEFR were significantly less sensitive in patients with both UAO and CAL than in those with UAO alone (35 vs. 85% and 52 vs. 72%, respectively). In all patients the most specific indices (100%) were FEF50/FIF50 and MVV/FEV1. The index MVV/FEV1 was the most accurate in patients with UAO and CAL. We conclude that when patients with CAL present 3 or more abnormal UAO indices, or have FEF50/FIF50 > or = 1 or MVV/FEV1 < or = 25, the possibility of simultaneous UAO must be strongly considered.