Wegener's granulomatosis is an uncommon type of vasculitis that causes inflammation of the blood vessels. This in turn restricts blood flow to various organs. It is distinguished from other forms of vasculitis by the presence of granulomas, which are not present in any other form of vasculitis. Most commonly Wegener's granulomatosis affects the kidneys, lungs and upper respiratory tract. When both lungs and kidneys are affected, the condition is sometimes referred to as generalized Wegener's granulomatosis. By restricting blood flow to these organs, damage and complications can result.

Besides inflamed blood vessels, Wegener's granulomatosis produces a type of inflammatory tissue known as granuloma, which is found around the blood vessels. Granulomas can destroy normal tissue. It is still unclear what causes Wegener's granulomatosis, but it is not an infection and it is not a type of cancer.  There has been no determination if the condition is genetic. Wegener's granulomatosis usually affects young or middle-aged adults.  Although it is uncommon in children, it can affect people at any age.