The more exhaustive the evaluation of couples with unexplained inferti
lity, the more likely is the opportunity for detecting the aetiologica
l factor responsible for infertility. Transport of spermatozoa through
the upper genital tract and their ability to fertilize the oocyte are
two obscure areas for the conventional evaluation of infertility. Alt
hough research in the former area is limited, there is indirect eviden
ce that impaired sperm transport could be one of the causes of inferti
lity in some couples with otherwise unexplained infertility. On the ot
her hand, the availability of sperm function tests and the correlation
of their results with in-vitro fertilization rates have allowed the d
etection of a previously hidden male factor in couples with unexplaine
d infertility. It has been demonstrated that couples suffering unexpla
ined infertility have significantly lower in-vitro fertilization rates
in comparison with patients with tubal problems. These results can be
explained because several case control studies in patients with unexp
lained infertility have reported defects in capacitation and sperm mot
ion characteristics, binding of the spermatozoa to the zona pellucida,
acrosome reaction, acrosin activity of the spermatozoa, and the abili
ty of the spermatozoa to penetrate zona-free hamster oocytes. These ob
servations suggest that methods for assessing the fertilizing capacity
of the spermatozoa have to be incorporated in the evaluation of coupl
es with unexplained infertility in order to amplify the scope of the w
orkup and to better decide the appropiate treatment for these couples.