A relevant set of lung function parameters, derived from spirometry, flow-volume curves, diffusion capacity and bodyplethysmography, to discriminate asthma from COPD was established via logistic regression analysis. All new patients, referred to the outpatient clinic and later defined as asthma or COPD, underwent extensive lung function testing with reversibility testing. Logistic regression was used to calculate the probability to be a COPD or asthma patient. Selection of relevant parameters was done via 1] forward-, 2] backward-, 3] stepwise selection and 4] the best score method. All four methods were supplemented by bootstrapping to obtain a validated selection and estimation of the logistic regression parameters. The area under the ROC curve (mean+/-sd) for respectively the forward, backward, stepwise and best score selection method is 0.9348+/-0.0115, 0.9346+/-0.0115, 0.9348+/-0.0115 and 0.9296+/-0.0121. The TLCO, VA and the postdilator MEF50, VC and PEF were selected as the most relevant parameters in discriminating asthma from COPD: they appeared most often as relevant discriminators in 500 bootstrapped samples: TLco was present in all bootstrapped samples and VA, postdilator MEF50, VC and PEF in resp. 70.8%, 46.2%, 42.8% and 36.8%. Bodyplethysmography derived parameters turned out to be of limited value. Diffusion capacity testing and spirometry/flow-volume curve after administration of bronchodilators are the methods of choicewhen having to chose between asthma or COPD.