Balsalazide (BSZ) is a pro-drug which releases 5-aminosalicylic acid (5ASA) and 4-aminobenzoyl-beta-alanine (an inert carrier) in the colon of various species including man. BSZ was compared with sulphasalazine (SASP) (both 1 g b.d. orally) in the maintenance of remission in patients with ulcerative colitis (UC). Seventy-nine patients (53 male, 26 female), mean age 49 years (range 19-79 years), with UC were randomly allocated to either treatment (41 BSZ, 38 SASP) for 6 months. The groups were similar in respect of age, sex, duration and extent of disease. Seven patients defaulted (3 BSZ, 4 SASP) leaving 38 on BSZ and 34 on SASP. Two male patients, both receiving SASP, were withdrawn because of severe side-effects. One of these patients, with an exfoliative rash, was maintained satisfactorily on open BSZ. Remission rates at 6 months (51% BSZ, 63% SASP) were not significantly different (life-table analysis P less than 0.1). Twelve patients (15%) reported troublesome side-effects (2 BSZ 5%, 10 SASP 26%, P = 0.017 Fisher Exact Test). Mean haemoglobin concentrations, similar on entry, increased after 6 months with BSZ (0.2 g/dl) but decreased with SASP (0.5 g/dl) (P less than 0.0002). BSZ was not significantly different from SASP in maintaining remission in patients with UC but had fewer side-effects.