The effect of different types of textile underpants on sexual activity was studied in 50 men. All the subjects were potent and sexually active. They were divided into 5 equal groups: 4 test and 1 control. Each of the 4 test groups were dressed in one type of textile underpants made of either 100% polyester, 50/50% polyester/cotton mix, 100% cotton, or 100% wool. Sexual behavior was assessed before and after 6 and 12 months of wearing the pants, and 6 months after their removal. Behavioral response was rated as potent if the subject's penis became erect, entered the vagina, and ejaculated. The rate of potent intromission (I) to mounts (M) (I/M ratio) was determined. The electrostatic potentials (EP) generated on the penis and scrotum were measured by an electrostatic kilovoltmeter. The I/M ratio at 6 and 12 months of wearing the polyester and polyester/cotton mix pants was significantly reduced compared to the pretest levels and the controls (p < .001). The reduction was more manifest in the pure polyester than in the polyester/cotton mix group, and at the 12-month than at the 6-month examination. The I/M ratio of the cotton and wool groups showed insignificant changes during the study period. Six months after removal of the underpants, the I/M ratio had returned to the pretest levels in the two groups that had worn polyester-containing pants. The polyester-containing pants generated EP, which may induce electrostatic fields in the intrapenile structures and could explain the diminished sexual activity. The cotton and wool textiles did not generate EP. Thus, polyester underpants could have an injurious effect on human sexual activity.