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We report on 15 families with familial primary spontaneous pneumothorax detected through a retrospective study of a large group of male patients. Genetic analysis of these families together with 14 families from the literature in which enough details were available suggested 2 possible models of inheritance. First, the familial cases are due to an autosomal dominant gene with incomplete penetrance, the penetrance being lower in females (21%) than in males (50%). Second, the familial cases represent a heterogeneous group of patients: some of the familial cases are due to an X-linked recessive gene and others to an autosomal dominant gene with the incomplete penetrance (35% in females and 50% in males). The second model may explain the excess of female carriers found in the families of the patients and is supported by the existence of clinical differences between the patients in each of the sub-groups. In particular the number of episodes of pneumothorax per patient was significantly higher in the sub-group with the autosomal dominant form than in the sub-group with the X-linked recessive form of the disease.