Treatment with the antimuscarinic agents tolterodine and oxybutynin is the mainstay of therapy for overactive bladder, a chronic and debilitating condition characterized by urinary urgency with or without urge incontinence, usually in combination with urinary frequency and nocturia. This study consisted of two trials; in one, patients with overactive bladder were randomized to 8 weeks of open-label treatment with either 2 mg or 4 mg of once-daily extended-release tolterodine (TER), and in the other to 5 mg or 10 mg of extended-release oxybutynin (OER). The study protocol and design were identical for the two trials and site selection ensured that there was no bias in either trial for the tendency of investigators to prescribe one drug rather than the other, or for geographical location. A total of 1289 patients were enrolled, 669 in the tolterodine trial (TER 2 mg, n = 333; TER 4 mg, n = 336) and 620 in the oxybutynin trial (OER 5 mg, n = 313; OER 10 mg, n = 307). Fewer patients prematurely withdrew from the trial in the TER 4 mg group (12%) than either the OER 5 mg (19%; p = 0.01) or OER 10 mg groups (21%; p = 0.002). More patients in the OER 10 mg group than the TER 4 mg group withdrew because of poor tolerability (13% vs 6%; p = 0.001). After 8 weeks, 70% of patients in the TER 4 mg group perceived an improved bladder condition, compared with 60% in the TER 2 mg group, 59% in the OER 5 mg group and 60% in the OER 10 mg group (all p < 0.01 vs TER 4 mg). Response to therapy was greater in a subgroup of patients whose perception of bladder condition was moderate to severe at baseline (TER 4 mg 77% vs OER 10 mg 65%; p < 0.01). Dry mouth was dose-dependent with both agents, although differences between doses only reached statistical significance in the oxybutynin trial (OER 5 mg vs OER 10 mg; p = 0.05). Patients treated with TER 4 mg reported a significantly lower severity of dry mouth compared with OER 10 mg. In conclusion, the greater efficacy and tolerability of tolterodine ER 4 mg suggests improved clinical effectiveness compared with oxybutynin ER 10 mg.