The Chernobyl accident (April 26, 1986) exposed a large part of the Belarus population to ionizing radiation. We analyzed the time trends of Down syndrome (DS) in Belarus to evaluate whether either brief exposure at high dose rates during the plume passage or continuous exposure at low doses and dose rates of the residents of contaminated areas had any detectable impact on DS prevalence at birth. DS data came from the Belarus National Registry of Congenital Malformations (1981-2001). We observed a significant peak of DS in January 1987 (26 cases observed and 9.84 expected; observed/expected ratio = 2.64; 95% CI = 1.72-3.76), but found no positive long-term time trends in contaminated or control areas. The time occurrence of the January peak, high dose rates during the plume passage and experimental data showing a radiosensitive phase of oogenesis around conception time in mammals suggest that the January peak may be linked to the Chernobyl plume.