Many of the arboviruses (arthropod-borne viruses) of public health importance in the United States and globally were discovered in the 1930s; yet, with few exceptions, little is known of their potential for vertical transmission to the fetus or as teratogens. The recognition of congenitally acquired infections may have been obscured in developing countries by high infant mortality rates, but, perhaps as importantly, where these infections are transmitted in an endemic pattern, acquired immunity may limit infections in females by the time they reach childbearing age. In these circumstances, the potential for a virus to cause intrauterine infections might not be detected until it is introduced into an immunologically naive population (or conversely, when an immunologically naive traveler to an area with endemic transmission is infected). The former situation occurred when Japanese encephalitis (JE), an endemic mosquito-borne flaviviral infection that has been recognized as a major cause of epidemic encephalitis in East Asia, possibly as early as the 19th century,1 was introduced into Northern India. JE viral infections occur ubiquitously in rural areas of the continent where viral transmission from a pig and rice paddy–breeding mosquito amplification cycle results in high immunity rates by adolescence in the rural population. Despite the high level of endemic infection and the documented occurrence of widespread outbreaks in Japan, China, and Southeast Asia since 1935 (when the virus was discovered), no cases of congenitally acquired JE infection were reported until 1980, when a series of epidemics in Uttar Pradesh, India, signaled the introduction of the virus into an area in which it had not been recognized previously. Among 9 reported cases that occurred during pregnancy, 4 led to miscarriage, and JE virus (JEV) was isolated from products of conception, including fetal brain, in 3 cases.2,3 Infection of the central nervous system … Address correspondence to Theodore F. Tsai, MD, MPH, Asia-Pacific Medical Affairs, Wyeth Research, Collegeville, PA 19426. E-mail: tsait{at}wyeth.com