There are few studies supporting the effective and safe use of laxatives for constipation. This study examined the short-term efficacy and safety of sodium picosulphate in patients with chronic constipation. Patients with a history of chronic constipation for at least 3 months were randomised to receive 7 mg sodium picosulphate or placebo for three consecutive nights. Patients recorded stool frequency and consistency, straining, bloating, and pain at baseline and during treatment. Vital signs, haematocrit, serum creatinine and electrolytes were monitored. Primary end-point for efficacy was the occurrence of a response to treatment, defined as improvement in stool frequency and occurrence of straining. All 57 randomised patients (sodium picosulphate n = 29, placebo n = 28; mean age 54.8 and 54.1 years) completed the study. Sodium picosulphate produced a treatment response (improved stool frequency and straining) in 82.8% compared with 50% in the placebo group (p = 0.010) and reduced bloating more often than placebo. There were no serious adverse events and one patient with diarrhoea and another with abdominal pain in each treatment group. There were no cardiovascular effects, changes in serum haematocrit, creatinine or electrolytes in either group. This study confirmed that sodium picosulphate is an effective, well-tolerated and safe laxative in the acute treatment of constipation.