A preliminary study reported finding higher sperm velocity in seminal plasma in males of partners that conceived female offsprings. The null hypothesis was that sperm velocity was not related to the offspring gender. The objectives were: (a) to expand the previous study, and (b) to correlate offspring gender results with motility parameters determined through the computer-aided sperm analyzer (CASA) system. In combined fresh and frozen cycles (N = 187), sperm from cases with all female offsprings displayed higher curvilinear (48 +/- 1.0 mu/sec versus male 46 +/- 1.0, P < 0.05) and average path velocities (36 +/- 0.7 mu/sec versus male 34 +/- 0.7, P < 0.01). A criteria of less than 30 mu/sec or over 41 mu/sec average path velocity predicted 73 or 72% of the male or female offspring cases, respectively. A curvilinear velocity of less than 49 mu/sec or over 55 mu/sec predicted 58 or 59 % of the male or female offspring cases, respectively. Semen viscosity reflected in sperm velocity was linked to predominantly male or female sperm populations. Paracrine signals from the gender-skewed sperm precursor populations controlling viscosity merit further exploration.