OBJECTIVE To investigate whether lycopene levels in blood and seminal plasma increase after dietary supplementation with a natural source of the compound, and whether any potential increase of lycopene levels in semen translates into increased free-radical trapping capacity in the seminal plasma. METHODS Reactive oxygen species are detrimental to the health and function of spermatozoa. Semen contains enzymatic and non-enzymatic defence mechanisms to combat such species, and lycopene, a dietary antioxidant, forms part of the non-enzymatic arm. Immuno-infertile men have significantly lower levels of lycopene in their semen, and oral lycopene therapy can improve various seminal variables in idiopathic infertility. Whether this improvement is a direct consequence of increased lycopene levels in semen, resulting in an increased radical scavenging ability, remains unknown. Blood and seminal lycopene levels were measured in healthy volunteers, using high-performance liquid chromatography, before and after a period of dietary supplementation. The antioxidant capacity of seminal plasma was also assayed to determine if supplementation results in a measurable increase in seminal radical scavenging ability. RESULTS There were statistically significant increases in blood and seminal plasma lycopene levels after dietary supplementation. The increase in seminal and blood lycopene levels showed a strong positive correlation (r = 0.84, P < 0.05). There was no measurable increase in the total radical scavenging capacity of semen. CONCLUSION This study confirms the presence of lycopene in human semen, the levels of which can be significantly increased after dietary supplementation with a natural source of lycopene. Further studies to establish whether this would also be the case in infertile men, with possible associated improvements in their seminal quality, are warranted.