The signs and symptoms of preeclampsia are usually apparent at a relatively late stage in pregnancy (late second to early third trimester). However, the disorder results from abnormal interaction between fetal and maternal tissue much earlier in pregnancy, between 8 and 18 weeks' gestation. During the past two decades numerous clinical, biophysical, and biochemical tests have been proposed for the early detection of preeclampsia. Some of these tests are simple, whereas others are invasive; some have been studied extensively, while others are still under clinical investigation. A review of the literature indicates considerable disagreement regarding the sensitivity and predictive values of the various tests studied. The reported differences in the predictive values of these tests may be attributed to one or more of the following: populations studied, definition and prevalence of the disorder, techniques and methodology used in performing these tests, etc. As a result, there is disagreement regarding the ideal screening test to be used for identifying patients for clinical trials dealing with prevention of the disorder.