In some male patients with findings characteristic of cystic fibrosis, but normal sweat chloride concentrations, the demonstration of obstruction azoospermia has been pivotal in diagnosis. It is proposed that cystic fibrosis be diagnosed if the patient has at least two of three major criteria (marked rise in sweat chloride concentration, chronic obstructive pulmonary disease and pseudomonas infection, and unexplained obstructive azoospermia) or has one major criterion together with one of several minor criteria--including positive family history and childhood onset of exocrine pancreatic insufficiency. Inclusion of unusual variants within the cystic fibrosis syndrome has major theoretical implications for pathogenesis and practical importance for genetic counselling and patient care.