A number of the systemic vasculitides (SV) may have an allergic component although specific allergens remain unknown. We evaluated the prevalence of several types of allergy in 60 consecutive patients with SV and their families. Two control groups were studied. One included 60 healthy people and the other 60 unselected patients attending a general rheumatology clinic who did not have SV. Forty-four of the 60 patients (73%) had a history of at least one type of allergy. This was significantly higher than in either control group (P = 0.0001 and P = 0.0002). There was also a significantly higher prevalence of allergies among the families of the SV patients compared to the controls. This data supports the idea that an allergic response to an external allergen may play an important part in the aetiology of these diseases.