Seminal and blood serotonin levels were measured in 37 men with varicocele and in 33 men without palpable varicocele. Elevated levels of blood serotonin and lower levels of seminal serotonin were found in oligozoospermic men with varicocele as compared with men having varicocele but normal seminal quality. No differences in seminal serotonin levels were observed in oligozoospermic men without varicocele. Elevated levels of blood serotonin and lower levels of seminal serotonin in semen in men with varicocele were associated with low sperm count, low sperm motility, low fructose levels, and high citric acid levels. It would appear that serotonin may play a role as an infertility determinant in subjects with varicocele. © 1990 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.