Diagnosis of occupational asthma is verified by inhalation challenge testing when frequent monitoring of peak expiratory flow (PEF) or forced expiratory volume in 1 s (FEV1) is needed. FEV1 measurements also allow frequent monitoring of changes in small airways. For the follow-up of lung function of suspected occupational asthma, a reliable, personal and mobile device is needed. We studied the accuracy of a pocket-sized spirometer, Micro Plus Spirometer (MP), compared with our former combination of two devices, i.e. the values of forced expiratory volume in 1 s (FEV1) with those measured with the Vitalograph bellow spirometer (V); and the peak expiratory flow (PEF) values with the values obtained with a Wright PEF meter (W). In healthy control subjects, the values of FEV1 obtained with Micro Plus correspond well to those obtained using a bellow spirometer (R = 0.97) and the mean difference (MP-V) was -0.06 1 irrespective of the mean FEV1 value. W gives smaller PEF values than the MP; the mean difference in PEF (MP-W) was 44 1 min-1, but the values increased linearly with increasing flow. The poor relation between PEF values might be based on the low reliability of the Wright device. In patient follow-up, the FEV1 values measured with MP and V showed a good correlation (R = 0.99). The PEF values in patients increase linearly measured with W vs. MP with increasing flow values. In conclusion, we found the pocket-sized spirometer a handy and useful device for monitoring bronchial challenge procedures in patients with suspected occupational asthma. The number of exhalations during the challenge test follow-up is lessened when FEV1 and peak expiratory flow (PEF) can be measured simultaneously.