Maca (Lepidium meyenii) is a herbaceous plant grown at over 4,000 m in Peru. It has been studied worldwide for its properties on fertility. Previous studies have assessed maca effects on semen quality, but there is need of randomised, double-blind trials in order to make clinical decisions. The objective of this study was to assess the effect of maca on seminal parameters in infertile adult men. This is a double-blind, randomised, placebo-controlled pilot trial in which sixty-nine patients diagnosed with mild asthenozoospermia and/or mild oligozoospermia were supplied by maca (n = 35) or placebo (n = 34) (2 g/day) for a period of 12 weeks. When compared patients treated with maca and patients treated with placebo, there were no significant differences in semen volume (2.95 ± 0.52 vs. 2.90 ± 0.52; p = .392), sperm motility (22.34 ± 2.22 vs. 23.05 ± 2.22; p = .462) and normal sperm morphology (7.89 ± 1.89 vs. 7.04 ± 2.28; p = .801), but there was a significant difference in sperm concentration (15.04 ± 5.61 vs. 10.16 ± 3.59, respectively; p = .011). In conclusion, patients treated with 2 g of maca for a period of 12 weeks showed a significant improvement in seminal concentration compared with patients treated with placebo. There were no significant differences in semen volume, sperm mobility and morphology when compared both groups.