Home monitoring of spirometry has been advocated in lung transplant recipients for the early detection of acute infection and rejection of the allograft. We have developed a user-friendly, Internet-based telemonitoring system providing direct transmission of home spirometry to the hospital. In this prospective study, we assessed patient adherence with the monitoring, agreement between home and hospital spirometry, intrasubject coefficient of variation (CV) for FEV(1) and FEF(25-75), and sensitivity of these variables for the detection of acute complications. Twenty-two bilateral-lung and heart-lung transplant recipients were followed for a median of 473 d (range, 60-822), during which 13,833 measurements were obtained. Patient compliance was 55% for two measurements a day and 84% for one measurement a day. Agreement between home and hospital spirometry was within 4% for FEV(1) and 6% for FEF(25-75). Mean CV was 3.2% for FEV(1) and 7.5% for FEF(25-75). Using transbronchial lung biopsy and/or bronchoalveolar lavage as gold standards, the sensitivity of home spirometry was 63%, and 23% of true positives were detected by changes in FEF(25-75) alone. We conclude that home monitoring of pulmonary function in lung transplant recipients via the Internet is feasible and provides very reproducible data; yet it has only a mild sensitivity for the detection of acute allograft dysfunction.