Microscopic haematuria is a common clinical finding, with reported prevalences of up to 22%. The role of renal biopsy in the investigation of this condition is still debated. Currently urological investigation including cystourethroscopy is often regarded as adequate. We investigated 165 patients (94 male, 71 female; mean age 37.5 years, range 10-71) referred with isolated microscopic haematuria, using renal biopsy and cystourethroscopy. All patients were normotensive with normal serum creatinine, no proteinuria, sterile urine and a normal IVU. Renal biopsy abnormalities were found in 77/165 (46.6%): IgA nephropathy (49), global or segmental mesangial proliferative glomerulonephritis without IgA deposits (16), thin membrane nephropathy (7), vascular changes suggestive of hypertension (3), interstitial nephritis (1), and membranous nephropathy (1). Only five abnormalities were found on cystourethroscopy (cystitis 3, urethral stricture 1, bladder stone 1). Two patients with cystitis also had IgA nephropathy. Biopsy abnormalities were commonest under the age of 20 (69.2%), but 40% of biopsies were abnormal even in the seventh decade of life. Because renal biopsy abnormalities are very frequent in patients with isolated haematuria, renal biopsy is indicated in patients over 45 years of age if renal imaging and cystoscopy are normal. In those under 45 years, renal biopsy should replace cystoscopy as the investigation to follow normal renal imaging.