BACKGROUND: The frequency of combined obstruction and restriction identified in pulmonary function tests has not been well described. Moreover, although the causes of combined-obstructionand-restriction patterns are known, the frequency of the various etiologies has received little attention. METHODS: We retrospectively reviewed medical records and surveyed pulmonologists. RESULTS: 43,212 PFT sessions were evaluated, which yielded 130 patients who satisfied our criteria for spirometry evidence of combined obstruction and restriction. Their demographic features were: mean ± SD age 54 ± 14 y, 51% male, mean ± SD body mass index 28.8 ± 6.7 kg/m2, mean ± SD height 174 ± 9 cm (men) and 162 ± 7 cm (women). The causes of combined obstruction and restriction were classified as either a pulmonary parenchymal disorder (Group A, n = 49, 38%) or a combination of pulmonary parenchymal and non-pulmonary diseases (Group B, n = 63, 48%). In 18 patients (14%) no clear etiology of combined obstruction and restriction could be determined. The most common pulmonary disease was chronic obstructive pulmonary disease (45/130, 35%), and the most common non-parenchymal disease was congestive heart failure (27/130, 21%). We electronically sent a survey to 55 pulmonary physicians, of whom 30 (55%) responded. The respondents estimated that combined obstruction and restriction occurs in approximately 20% of all the pulmonary function tests performed in their practices and that pulmonary parenchymal diseases were responsible for 35% of all instances of combined obstruction and restriction. CONCLUSIONS: Combined obstruction and restriction occurs infrequently and is more commonly caused by a combination of pulmonary parenchymal and non-pulmonary disorders. Pulmonologists' impressions regarding the frequency and causes are generally discordant with the observed frequencies. © 2010 Daedalus Enterprises.