To explore the feasibility and validity of forced spirometry in patients with ataxia telangiectasia (A-T). Twenty-eight patients (aged 3.7-19.3 years) performed spirometry on 47 occasions. Parameters studied were technical quality and relation to: predicted values, pulmonary illness. Start of test criteria for correct expiratory effort was significantly prolonged (183 ± 115 ms; P < 0.001). The rise-time to peak flow in children free of respiratory symptoms (Group-FRS; n = 8) increased by 16.2 ± 12.5 ms/year above recommended and in children having recurrent infections (n = 8) 30.4 ± 16.1 ms/year, P < 0.01. Expiration-time was significantly shorter than requested (1.21 ± 0.47 sec) and was ended abruptly in 57% of the patients. FEV(1) could not be established by 8/20 patients. The intra-subject reproducibility met criteria (4.4 ± 2.7%, 5.2 ± 2.8%, 2.9 ± 3.2%, 6.3 ± 5.3%, for FVC, FEV(0.5), PEF, FEF(25-75), respectively). Group-FRS showed yearly deterioration in FVC of 2.2%, while patients with hyper-reactive airways (Group-HRA; n =12) had a deterioration rate of 3.6%/year. FEV(0.5) deterioration rate was similar in both groups (2.2 and 2.0, respectively), but baseline values in Group-HRA were significantly lower than those of Group-FRS (P = 0.029) in similar young ages, indicating airway obstruction at early ages in Group-HRA. FEV(0.5) values deterioration also correlated with body mass index (P< 0.017). Forced spirometry in A-T patients is reproducible and has a distinct pattern, although curves do not meet other recommendations for acceptable criteria. The study insinuates that a rapid deterioration in lung function occurs in A-T patients with recurrent respiratory infection, suggesting that early intervention may prevent further deterioration or improve their lung function. Further studies are needed to confirm our results.