A nested case-control study of cryptorchidism (i.e. undescended testicles) was undertaken as part of a hospital-based cohort study of 6699 singleton male neonates in New York City. Since some of the cryptorchid infants experienced spontaneous descent of their testes, separate analysis was performed for this third group of 'late descenders' (n=140). Cases (n=63) represented infants whose testes remained undescended at the one year assessment. Controls (n=219) represented the next male infant who was delivered immediately after an infant who was cryptorchid at birth. The only independent risk factors for cryptorchidism were Asian ethnic group (adjusted odds ratio (OR) = 3.90, 95% confidence interval (CI) = 1.22-12.41), swollen legs or feet during pregnancy (adjusted OR = 2.16, 95% CI = 1.15-4.04), a family history of cryptorchidism (adjusted OR = 4.32, 95% CI = 1.91-9.80), low birthweight (adjusted OR = 4.10, 95% CI = 1.39-12.08), and use of analgesics during pregnancy (adjusted OR = 1.93, 95% CI = 1.03-3.62). Multiple logistic regression analysis was also performed to identify those factors that were associated with late testicular descent. In this analysis the independent risk factors were black or Hispanic ethnicity (adjusted OR = 2.05, 95% CI = 1.09-3.83), a family history of cryptorchidism (adjusted OR = 4.25, 95% CI = 1.84-9.78), consumption of cola-containing drinks during the pregnancy (adjusted OR = 2.09, 95% CI = 1.10-3.99), a low birthweight delivery (adjusted OR = 9.78, 95% CI = 3.39-28.20), and preterm birth (adjusted OR = 4.01, 95% CI = 1.66-9.70).