Restrictive lung disease comes in two major categories: (1) intrapulmonary (= parenchymal) disease caused by fibrotic reactions or (2) extrapulmonary (= compression), like in heart failure. In the first category the conducting airways, tethered in stiffened structures, are less likely to be compressed during forceful expiration and expiratory flows hence are expected to remain high. This could serve as a cheap and easy diagnostic, avoiding more complicated measures. A database was build containing 624 patients suffering from either intra- and extrapulmonary disease. The flow-volume curve indices of restrictive patients (with a total lung capacity < -1.96 sd of reference) were compared and it was shown that in primary fibrotic disease and in leukaemia, indeed, the PEF and MEF(75/50/25) were significantly higher compared to the heart failure group (P < or = 0.001). The diabetes mellitus vs. heart failure differences were much less (P > 0.05). The area under the ROC to discriminate extra- from intrapulmonary disease was a low 0.607 and 0.606 for the PEF and MEF75, respectively. For the peakflow an optimal cut-off point was found at 65.8% of the reference value. The positive/negative predictive value of a peakflow < 65.8% to detect extrapulmonary disease was 30.1% and 82.2%, respectively.