The ratio between the second and fourth fingers (2D:4D) is supposed to serve as a putative indicator of prenatal testosterone (PT). Significant associations between 2D:4D, children's basic numerical ability, and the Spatial Numerical Associations of Response Codes (SNARC) effect have recently been reported. The present study explored potential relationships between 2D:4D and the basic numerical ability of subitizing (the rapid enumeration of small quantities) in 80 right-handed adult volunteers. Participants completed a short battery of computerized subitizing and color recognition control tasks with both left and right hands, independently (order counterbalanced). Findings revealed a significant interaction between sex and 2D:4D on reaction time differences for right vs. left hand responses to the subitizing task. While 2D:4D in women showed a significant negative association with a right-hand advantage for the task, a nonsignificant trend in the opposite direction was observed for men. Results are discussed with respect to the possible effect of PT on sex differences in lateralization for basic quantification.