The potential benefits and pitfalls of the estrogen-creatinine ratio are discussed. Preference is expressed for complete 24-hr urine estrogen determinations as a means of serially assessing fetoplacental function for correlation with the clinical status of the pregnancy under study. However, the estrogen-creatinine ratio does show definite progression throughout pregnancy and there are situations in which an estrogen-creatinine ratio may be of value in lieu of a complete 24-hr collection.