Inguinal herniorrhaphy is the most common general surgical procedure performed on the neonate or young pediatric patient. The vas deferens and epididymis are vulnerable to damage, including transection during inguinal exploration or hernia repair. Occasionally the surgical pathologist encounters glandular or tubular epithelial-lined structures in hernia sac tissue. Significant medicolegal implications arise when embryonal remnants are mistakenly identified as true vas deferens or epididymis. This study evaluates the incidence and morphology of these embryonal remnants in hernia sacs from patients of The Children's Hospital, Denver, CO. Embryonal remnants were found in 1.5% of 599 hernia sacs from 427 consecutively operated males aged 37 weeks gestation to 19 years. True vas deferens and epididymis were each identified once for an incidence of 0.33%. The remnants resemble either vas deferens or epididymis. The average remnant diameter is 0.17 mm, and did not change significantly with age. Remnants are surrounded by varying amounts of condensed mesenchyme, trichrome-negative for muscle. The testes and vasa differentia from 32 autopsy cases, ranging in age from 26 weeks gestation to 7 years of age, were used to evaluate normal development and morphology of the vas deferens, epididymis and embryonal remnants. The vas deferens is well developed by 26 weeks gestation. The surrounding smooth-muscle coat does not stain with trichrome until 32 weeks gestation. The vas deferens increases in diameter in a linear fashion during gestation, and continues to increase in diameter in the postnatal period. The vas deferens at 4 months of age is 1.2 to 1.4 mm in diameter; this is also the age of highest incidence of bilateral herniorrhaphies and presence of remnants.