An underactive urinary bladder (UUB), often occurring after surgery, can lead to urinary retention even in otherwise healthy people. We systematically reviewed published reports to determine whether the use of parasympathomimetic agents is warranted in patients with a UUB. Agents allegedly useful in treating UUB were identified from urology and pharmacology textbooks. A systematic search for randomized clinical trials in patients with UUB using these agents revealed 10 such studies. Controls typically received placebo or no treatment. While three studies reported statistically significant improvements relative to the control group, six did not and one even reported a significant worsening of symptoms. There was no evidence for differences between individual drugs, specific uses of such drugs, or in outcome measures. We conclude that the available studies do not support the use of parasympathomimetics for treating UUB, specifically when frequent and/or serious possible side-effects are taken into account.