['To clarify whether fibulins-5 is associated with primary spontaneous pneumothorax (PSP) in young PSP patients.', 'Forty-six surgically resected, fresh lung specimens were used. Patients were divided into 3 groups: younger than 25 years with pneumothorax (group Y), 25 years or older with pneumothorax (group O), and without pneumothorax (group C). Chest X-ray, computed tomography data, height/width ratio (H/W) and anteroposterior/transverse diameter ratio (a/b) were measured. Elastica van Gieson staining and immunofluorescence staining for fibulin-5 were performed. Fibulin-5 mRNA expression and protein levels were measured by real-time PCR and western blotting. Direct sequences of the fibulin-5 gene in PSP patients were performed.', 'The mean H/W ratio in group Y was significantly larger than that in the other groups (p <0.01). The mean a/b ratio in group Y was significantly smaller than that in the other groups (p = 0.02). Fibulin-5 mRNA expression was not significantly different among the groups (p = 0.64). The relative intensity of fibulin-5 protein in group Y was significantly lower than that in group O (p = 0.006), with no significant differences between groups O and C (p = 0.14).'] Is fibulin-5 protein reduced in the lung of patients with spontaneous pneumothorax who are under 25 years old?