The prevalence and causes of microscopic haematuria were examined in all Type 1 (insulin-dependent) diabetic patients with persistent proteinuria (diabetes duration greater than or equal to 5 years) attending the outpatient clinic at Hvidöre Hospital during 1985. One hundred eighty-four patients (69F/115M) out of 1024 Type 1 patients had persistent proteinuria (18%). Microscopic haematuria was defined as greater than or equal to 3 erythrocytes per high power field in two or more sterile urine samples. Twenty-three Type 1 patients with persistent proteinuria (7F/16M, aged 35.4 +/- 13 years) had microscopic haematuria (12.5%). No significant changes were found between the group with and without microscopic haematuria: blood pressure 148/89 +/- 22/11 versus 145/91 +/- 20/11 mmHg, duration of diabetes when persistent albuminuria occurred 17 +/- 8 versus 20 +/- 10 years, serum creatinine 99 +/- 24 versus 98 +/- 31 mumol/l, simplex retinopathy 61 versus 54%, proliferative retinopathy 39 versus 42%, and no signs of retinopathy 0 versus 4%. Kidney biopsy was performed in 13 out of the 23 patients with microscopic haematuria. Diabetic glomerulosclerosis was present in all 13 patients, but 9 patients had a non-diabetic renal disease superimposed (mesangioproliferative glomerulonephritis (n = 5), membranous glomerulonephritis (n = 3) and sarcoidosis (n = 1). Microscopic haematuria is a rare finding, frequently reflecting superimposed non-diabetic glomerulopathies, in Type 1 diabetic patients with diabetic nephropathy and well preserved kidney function.