Renal abnormalities have been reported in Ankylosing Spondylitis (AS) patients. Possible mechanisms include the effects of nonsteroidal anti-inflammatory drugs (NSAIDs), an increased incidence of glomerulonephritis, particularly the ones associated with deposition of IgA-containing immune complexes and the renal deposition of amyloid. These observations prompted us to evaluate in detail the frequency and severity of renal dysfunction in 40 AS patients, consecutively selected attending the rheumatology disease unit outpatient clinic at Escola Paulista de Medicina, using sensitive tests of glomerular and tubular function. Fourteen of the 40 patients presented one or more signs of renal involvement: microscopic hematuria (9 patients), microalbuminuria (4 patients), decreased renal function assessed by serum creatinine (2 patients), and creatinine clearance (4 patients). None of the patients presented increased urinary excretion of retinol binding protein (RBP). The finding of renal abnormalities in 35% of our patients suggests that in this illness evidence of renal involvement should be actively investigated.