In this study, we determined the outcome in cases of isolated nuchal lucency seen sonographically in the first trimester in fetuses without karyotypic abnormalities. We reviewed all cases of isolated localized fetal nuchal lucency (3 mm or greater) in 9 to 14 week fetuses over a 4 year period. Fetuses with additional sonographic abnormalities were excluded. The width of the nuchal lucency at initial sonogram as well as findings on subsequent scans were tabulated. Karyotypic, pathologic, and clinical follow-up data were obtained. Of 44 fetuses with an isolated, localized first trimester nuchal lucency, one was lost to follow-up and two were excluded owing to pregnancy termination without karyotype or pathologic analysis, thus resulting in 41 fetuses in our study group. Five fetuses (12%) had abnormal karyotypes. Twenty-seven of the remaining 36 fetuses had normal karyotypes, eight others showed no evidence of aneuploidy at birth, and one patient underwent spontaneous abortion prior to a karyotypic analysis. Among the 36 fetuses without evidence of aneuploidy, six had a poor outcometwo were spontaneous abortions, one was a therapeutic abortion of a fetus with hydrops and a pericardial effusion seen on fetopsy; one fetus died at birth of pulmonary hypoplasia associated with autosomal recessive polycystic kidney disease, and one fetus each had Noonan syndrome, and Joubert syndrome. In addition, three patients delivered their infants prematurely. Overall, 32 of 41 fetuses survived, and two (6%) were abnormal. Excluding premature infants, 27 were normally grown, term survivors. We conclude that other than having an increased risk for aneuploidy, fetuses with isolated nuchal lucency are also at risk for spontaneous miscarriage, premature delivery, and congenital anomalies unassociated with an abnormal karyotype.