Purpose: To investigate the therapeutic effects and safety profile of antimuscarinics in patients with an overactive bladder (OAB) using the Indevus Urgency Severity Scale (IUSS) and uroflow parameters. Materials and Methods: Patients with urinary frequency (≥8 micturitions/day) and urgency, with or without urgency incontinence who were treated with antimuscarinic drugs (tolterodine SR 4 mg QD or oxybutynin 7.5 mg QD) for at least 1 month were enrolled. Patients were classified as having clinical benign prostatic hyperplasia (BPH) with OAB, idiopathic detrusor overactivity (DO) (IDO), and neurogenic DO (NDO). The IUSS, bladder capacity, voided volume, maximal flow rate (Qmax), and post-voiding residual (PVR) were recorded at the baseline and after 1 month of treatment. Adverse events were also recorded. Results: In total, 133 patients including 26 women and 107 men were enrolled. The IUSS (2.92±0.87 vs. 1.52±0.93, p=0.005) score significantly decreased and Qmax (13.9±8.2 vs. 16.1±13.4, p=0.036), bladder capacity (276.7±162.9 vs. 313.1±176.6, p=0.009), and PVR (65.0±68.42 vs. 87.7±104.5, p=0.008) significantly increased after treatment. The voided volume, corrected Qmax (CQmax), and voiding efficiency did not significantly change after antimuscarinic treatment. Analysis of different OAB groups showed that the benign prostatic hyperplasia (BPH) with OAB, IDO, and NDO groups had similar results. All measured parameters except voided volume, CQmax, and voiding efficiency significantly changed after treatment. Recorded moderate to severe adverse effects included urinary retention in 2 patients (1.5%), dry mouth in 8 (6.0%), blurred vision in 2 (1.5%), constipation in 7 (5.3%), and edema in 1 (0.8%). Seven patients (5.2%) discontinued treatment within 1 month due to severe adverse effects in 6 and a poor drug response in 1. Conclusions: Antimuscarinic agents were well tolerated and safe in OAB patients without BPH or with well-treated BPH and had no significant effects on uroflow or voiding efficiency.