On the basis of the morphologic and hemodynamic features in 17 patients with bilateral bronchiectasis, a new subclassification is proposed. Accordingly, two types of bronchiectasis were recognized: perfused and nonperfused. Whereas perfused bronchiectasis has intact pulmonary artery flow and cylindrical bronchiectatic changes, the nonperfused type involves an absent pulmonary artery flow, retrograde filling of the pulmonary artery through the systemic circulation, and cystic bronchiectatic changes. A policy of unilateral resection of nonperfused bronchiectasis and preservation of the perfused type was adopted in 17 patients with bilateral bronchiectasis during an 8-year period. There were 9 women and 8 men with an average age of 28.6 +/- 7 years (range 18 to 48 years). Fifteen patients had mixed bronchiectasis (perfused type on one side and nonperfused on the other side) and two had localized bilateral nonperfused type. The average duration of follow-up was 38.3 +/- 24.9 months (range 13 to 111 months). In the 15 patients with mixed bronchiectasis, excellent (N = 8) or good (N = 7) results were achieved in all cases. On the other hand, the two patients with bilateral nonperfused bronchiectasis did not benefit from unilateral resection. This outcome implies that with perfused bronchiectasis the deranged function is likely to resolve with time. In the face of the general criticism of the traditional morphologic classification system, the proposed functional classification not only reflects the degree of severity of the disease process, but also predicts whether the involved lung will have a measure of respiratory function with regard to gas exchange. Thus the question of which side to resect and which to preserve is defined more precisely.