We studied the effects of levodopa (L-dopa), which is reported to increase the dopamine level in the brain, on male erectile function. Of the 21 subjects studied, 12 subjects who were 50 years old or older showed significant increases of two nocturnal penile tumescence (NPT) parameters, NPT frequency and total tumescence time, with L-dopa. On the other hand, in nine subjects who were younger than 50 years, maximum penile circumference increase showed a significant increment with L-dopa. This significant increment of NPT with L-dopa was not observed in the subjects who had low androgen levels. The results of this preliminary study show a positive relationship between administration of L-dopa and erectile function. L-Dopa administration may improve erectile function in subjects aged 50 years and older who have normal androgen levels.