T HE EFFECT of virus infections on the outcome of pregnancy has been of interest to investigators since Gregg (1) demonstrated a relationship between congenital cataract and German measles in the mothers. Following Gregg's observation in 1941, several investigators have attempted to associate virus infections with congenital abnormalities, prematurity, fetal deaths, or maternal deaths. Swann (2) in 1943 implicated maternal mumps as a cause of fetal anomalies. Others (3-15) have since reported on the influence of virus infections on both the fetus and the mother, with contradictory findings as to the effect of influenza infection. As a result of some of these investigations, pregnant women are generally considered to be a high-risk group with respect to influenza. Eickhoff and co-workers recommended that "immunologic protection through the routine use of influenza vaccine in such high-risk groups may be of great value in reducing the extent of influenza-associated excess mortality" (15). The importance of this recommendation emphasizes the need for critical evaluation and further studies. From August 1957, when the A2 strain of influenza virus first appeared in New York City, througlh June 1961, we were able to identify six periods of increased activity of the A2 influenza virus in New York City. We thus were offered an unusual opportunity to study the relationship between influenza and complications of pregnancy including fetal outcome in a large population during several outbreaks. This paper reports findings pertaining to maternal deaths, prematurity, fetal deaths, and congenital anomalies.