The point that host defense against infectious disease is exceedingly complex scarcely needs further emphasis. We have tried to show that pregnancy is a state which has the potential for altering any of the many facets of host defense, and in some instances presented evidence that pregnancy does alter resistance to infection. Some diseases are either more severe during pregnancy or more prevalent during pregnancy, suggesting a softening of the host defenses. However, both monocytic and polymorphonuclear leucocyte phagocytic systems seem to be more active during pregnancy. Transferrin is increased in pregnancy. The immune system in pregnancy is either normal or slightly depressed. A number of other factors have been mentioned for which no information exists as to the effect pregnancy has on them. Despite the information reviewed, it is not possible to say that pregnancy has a net positive or negative effect on host defense; the effect, if any, depends ostensibly on the disease entity. That the host defenses may be somewhat enhanced by pregnancy while susceptibility to certain diseases is increased is paradoxical and serves to underscore the fact that our understanding of host defense in pregnancy is abysmally deficient. Probably no concept is made more clear by this paper than the idea that in the area of host defense in pregnancy there is as much space as any researcher may choose to fill.