The causes of hydrops fetalis are myriad. As a result of the advent of routine Rh screening, most cases are not currently related to Rh incompatibility. Genetic, metabolic, chromosomal, and syndromic causes are among the most frequently identified causes of nonimmune hydrops. The importance of determining the underlying cause of hydrops becomes evident once issues such as prognosis, specific treatment, and risk of recurrence are considered. The medical geneticist is highly qualified to assist in the evaluation of hydrops. Clinical geneticists have undergone training in a primary care specialty followed by intensive training in the diagnosis, management, and counseling of individuals and families with genetic, chromosomal, or multifactorial syndromes or birth defects. This training prepares the medical geneticist well to serve as a consultant when hydrops is diagnosed. As knowledge of the molecular genetic and metabolic basis of disease increases, utilization of genetics laboratories continues to increase dramatically. In addition to examining the child to look for dysmorphic features, the clinical geneticist can assist with the laboratory evaluation by coordinating testing with the cytogeneticist, molecular geneticist, and biochemical geneticist as appropriate. Increased awareness of the role of the geneticist in the evaluation of such patients should prove helpful to the physicians caring for such patients and the patients' families.