Although the cause of nausea and vomiting of pregnancy is not known, there is strong evidence linking human chorionic gonadoptropin or estrogens. Evidence is presented to show that the incidence and severity of nausea and vomiting of pregnancy is linked to temporal and pathologic alterations in these hormones during pregnancy. The way in which the pregnant woman responds to the primary stimulus to nausea and vomiting of pregnancy appears to depend on her susceptibility mediated by vestibular, gastrointestinal, olfactory, and behavioral pathways. Conceiving of nausea and vomiting of pregnancy as a syndrome suggests new pathways of investigation and possible therapies.