Levels of circulating CD34⁺ cells in control subjects and in patients with chronic lung disease. Peripheral blood CD34⁺ cells were significantly lower in patients with idiopathic pulmonary fibrosis (n = 8) and lung fibrosis associated with connective tissue disorders (n = 7) on long-lasting oxygen therapy than age- and sex-matched control subjects free from clinical and instrumental signs of pulmonary and hematologic diseases (n = 45): mean ± SEM 0.051% ± 0.0024% versus 0.026% ± 0.003% of circulating leukocytes, respectively (P < .001). CTRL indicates controls; CLD, chronic lung diseases.