The airway walls of patients who have asthma or chronic obstructive pulmonary disease (COPD) are thickened by an increase in the amount of smooth muscle and connective tissue. Platelet-derived growth factor (PDGF) is a candidate cytokine for this increase because it can produce smooth muscle proliferation in vitro. The present study was designed to examine the expression of PDGF and its receptor (PDGFR) in lungs from six asthmatics, six patients with COPD, and six patients with normal lung function. PDGF was immunolocalized to tissue macrophages, but the number of PDGF-positive cells was similar in all three groups. PDGFR-beta was rarely expressed on interstitial cells, and, occasionally, on bronchial epithelium. Northern blotting, performed on tissue from the same groups, showed a positive correlation of PDGF(B) with PDGFR-beta mRNA level (r = 0.74, P < 0.001) and a higher abundance of PDGF(B) and PDGFR-beta mRNA in the asthmatics vs. the COPD (P < 0.05). We conclude that PDGF and its receptor are expressed in human lungs but do not correlate closely with the structural changes in diseased airways.