BACKGROUND: Delivery of a boy has been reported to increase a woman's risk of recurrent miscarriage in subsequent pregnancies. We explored whether delivery of boys similarly increases the risk of a subsequent stillbirth. METHODS: We identified all Danish women delivering their first child (singleton) between 1980 and 1998 (n = 499,731) using the Danish Birth Registry. These women had subsequent singleton births through 2004 (n = 558,314). We assessed the risk of stillbirth conditional on sex of prior children. RESULTS: The risk of stillbirth was increased by 12% after deliver of boys compared with girls (relative risk = 1.12 [95% confidence interval = 1.02-1.23]). This association did not appear to be explained by maternal confounders. CONCLUSION: Stillbirth risk appears to be slightly higher among the pregnancies of women who have previously delivered a boy. One possible mechanism is maternal immune response to male-specific minor histocompatibility antigens initiated during pregnancies with boys.