Analysis of 87 cases of xanthogranulomatous pyelonephritis ( XPN ) from 1958 to 1983 (14 males, 73 females, ages 13-85) revealed an incidence of 1.4 cases/100 000 population per annum which is apparently increasing. Clinical, radiological and pathological investigations demonstrated universal urinary obstruction (77.5 per cent calculi, 17.5 per cent pelviureteric junction obstruction) and pathogenic organisms such as E. coli or Proteus were cultured from the urine in 72 per cent of cases. Rare complications included sinuses or fistulae to bowel. We believe that the combination of urinary obstruction and infection by organisms of low virulence initiate XPN , and that associated lipid is derived from renal pelvic adipose tissue. Problems of differential diagnosis are discussed in relation to the use of immunocytochemistry and electron microscopy.