We have characterised nitric oxide (NO) synthase activity in lung samples from patients with inflammatory lung disease compared to that in normal donor lung. NO synthase activity was measured by the ability of tissue homogenates to convert L-arginine to L-citrulline. Higher levels of NO synthase activity were found in samples from patients with inflammatory lung disease (mild asthma, cystic fibrosis, obliterative bronchiolitis after lung transplantation) compared to samples from healthy donors. NO synthase activity in all samples was mainly dependent on the presence of extracellular Ca2+. The increased NO synthase activity in diseased-lung samples suggests a modulatory role for nitric oxide in lung inflammation.