Introduction Oral treatment with phosphodiesterase type 5 inhibitor (PDE5) is considered the first-line treatment for patients with erectile dysfunction (ED). Lodenafil carbonate (LC) is a novel PDE5. Aim This is a phase II, prospective, randomized, double-blind, and placebo controlled clinical trial of LC. Main Outcome Measures Efficacy end points were International Index of Sexual Function (IIEF) erectile domain, IIEF questions 3 and 4, and Sexual Encounter Profile (SEP) questions 2 and 3, before and after the use of LC or placebo. Methods Seventy-two men older than 18 years, with ED for at least 6 months with stable sexual relationship were enrolled. Patients were randomized to placebo or LC 80 mg, 40 mg, or 20 mg and followed for 4 weeks. Results IIEF erectile domain scores before and after the use of medications were (mean ± standard deviation [SD]): placebo: 11.9 ± 3.4 and 12.6 ± 5.5; LC 20 mg: 15.8 ± 4.1 and 18.9 ± 6.6; LC 40 mg: 11.9 ± 4.4 and 15.4 ± 8.1; LC 80 mg: 14.2 ± 4.7 and 22.8 ± 6.0 (anovaP < 0.01). The SEP-2 scores before and after the use of medications were (Mean ± SD): placebo: 71.0 ± 33.1 and 51.2 ± 43.1; LC 20 mg 70.3 ± 34.2 and 75.5 ± 31.5; LC 40 mg: 48.4 ± 42.1 and 60.8 ± 42.5; LC 80 mg: 68.6 ± 33.5 and 89.6 ± 26.0. The SEP-3 scores were: placebo 23.3 ± 27.6 and 33.6 ± 42.3; LC 20 mg: 32.3 ± 38.9 and 51.2 ± 41.7; LC 40 mg: 39.7 ± 44.7 and 46.7 ± 41.1; LC 80 mg* 17.2 ± 29.5 and 74.3 ± 36.4 (*P < 0.05 for difference to placebo). Conclusions The drug was well tolerated. Adverse reactions were mild and self-limited and included headache, rhinitis, flushing, color visual disorders, and dyspepsia. This study showed that the dosage of 80 mg of LC was significantly more efficacious than placebo and well tolerated.