The different results that have been published concerning the problem of the final outcome for patients who have had influenza during pregnancy reported in numerous works might be explained by differences in methodology used by the different authors. A study was carried out in the Haguenau Maternity Hospital (Bas-Rhin) when an epidemic of influenza occurred in 1972-173. The virological diagnosis was made by carrying out separate serological estimations on 1940 pregnant women. It can be shown that the mean birthweight dropped in infants of mothers who had contracted influenza during pregnancy even when corrections were made for sex and the duration of the pregnancy. The drop in weight of the placenta (37.3 g as a mean) was more obvious and can totally explain the drop in fetal weight. It was not possible to demonstrate any increase in the number of congenital malformations that were found in the neonatal period in the infants born to women who had had influenza. These results suggest that there is no direct passage of the influenza virus across the placenta which, however, is itself modified by the infection.