Thirty-six cases of Wegener's granulomatosis (WG) and microscopic polyarteritis (MPA) presenting to the nephrology service in Leicester between 1980 and 1989 were reviewed. Apart from the diagnostic respiratory tract lesions seen in WG, cases of MPA and WG could not be distinguished by age and sex, range and severity of organ involvement, response to treatment (oral prednisolone and cyclophosphamide), mortality or renal outcome. The combined incidence of WG and MPA in 1980-86 was 1.5/million/year. Following the introduction in January 1987 of an assay for anti-neutrophil cytoplasmic antibody (ANCA), the incidence of WG and MPA increased in 1987-89 to 6.1/million/year (p less than 0.0001). Cases seen during these two periods did not differ by any clinical parameter except that later cases had less severe renal disease at referral and improved renal outcome. Median serum creatinine was significantly lower at presentation in 1987-89 (p less than 0.02). Of those surviving 3 months from presentation only 1/20 in 1987-89 had end stage renal failure compared with 4/10 in 1980-86 (p less than 0.02). These findings may reflect increasing diagnostic awareness of WG and MPA among physicians since the introduction of ANCA.