To date there are only few reports evaluating the potential nephrotoxic reactions of the new 5-aminosalicylic acid (5-ASA) preparations in patients with ulcerative colitis (UC). The aim of this study was to screen the tubular and glomerular functions in patients with UC in maintenance treatment with either 5-ASA azo-compounds (sulphasalazine and olsalazine) or mesalazine. Patients with UC in clinical remission treated with either sulphasalazine, olsalazine, or mesalazine for more than 1 year were included in an open, single-blind retrospective Norwegian multicenter study. Serum and urine creatinine, serum and urine beta2-microglobulin, urine N-acetyl-beta-glucoseamidase (NAG), urine alkaline phosphatase, urine microalbumin, urine alanine amino peptidase, and urine beta2-microglobulin were measured. Fifty-two females and 75 males (n = 127), ages 20-69, were evaluated. Thirty-six patients were treated with sulphasalazine (mean treatment time 10.1+/-6.6 years [mean +/- SD]), 32 patients were treated with olsalazine (2.3+/-1.4 years), and 59 patients with mesalazine (3.2+/-2.0 years). At inclusion, there were no significant differences in the serum or urine values between the groups. In 17 patients (1 patient [3%] in the sulphasalazine group, 4 patients [13%] in the olsalazine group, and 12 patients [20%] in the mesalazine group), at least one abnormal serum and/or urine value was detected. After 10 years of treatment, only one abnormal value was found among the 19 patients in the sulphasalazine group. The abnormal values observed in the other groups indicated minor glomerular or tubular renal damage. In conclusion, long term sulphasalazine treatment appears to be safe and free of nephrotoxic side effects, whereas minor glomerular and tubular impairment are observed in a few patients treated with olsalazine and mesalazine.