Mucous metaplasia is an important determinant of small airway obstruction in COPD. Its relationship to small airway inflammation is poorly defined. We analyzed 4 to 6 small airways in 19 COPD patients, GOLD stages 0-4, from lobectomy or lung volume reduction surgery tissue samples. To identify intracellular mucin, periodic acid fluorescent Schiff's (PAFS) stained slides were imaged by fluorescence microscopy. PAFS+ staining area, basement membrane length (L(BM)), epithelial height and area were measured. Mucin was expressed as a percentage of epithelial area. Mucin volume density (MVD) was calculated as PAFS+ area divided by the product of L(BM) and 4/pi. Airways were Giemsa stained for eosinophils and immunostained with antibodies against CD3, CD4, CD8, CD68, and neutrophil elastase (NE), and the number of positively stained cells/mm(2) was quantified in the airway wall. Mucin percent correlated with CD3(+) cell density (r = 0.553, P < 0.0001), and MVD correlated with CD3(+) (r = 0.570, P < 0.0001) and CD8(+) cell density (r = 0.279, P = 0.016). There were weak negative correlations between mucin percent as well as MVD and CD68(+) cell density (r = -0.270, P = 0.02 and r = -0.245, P = 0.036). There was no relationship between epithelial mucin content and CD4(+), NE(+), or eosinophil cell density. CD3(+) and CD8(+) lymphocytic inflammation is related to small airway mucous metaplasia in COPD and may play a causative role in its development.