Doppler color flow mapping is widely used in fetal echocardiography. We studied the impact of color flow mapping on fetal cardiac diagnosess. Between January 1, 1989 and June 30, 1990, we performed 854 fetal echocardiograms on 776 fetuses. Color flow mapping was used in 45 of 48 fetuses diagnosed as having cardiac abnormalities. Scans were reviewed to assess how color flow mapping influenced the ultimate diagnoses. Color flow mapping was essential to the correct anatomical diagnoses in 13 fetuses (29%), helpful but not essential in 21 (47%), and added little to two-dimensional examination alone in 11 (24%). It was essential in determining the course and flow direction in the great vessels when outflow obstruction was present or with transposition, and it was helpful but not essential in locating small jets of atrioventricular valve regurgitation. It was not helpful when the anatomical abnormalities were clearly identified from two-dimensional examination alone. We conclude that color flow mapping is helpful in the delineation of anatomical diagnoses in three-quarters of cases of fetal heart disease, particularly when the great vessels are abnormal. It may speed examinations by directing pulsed Doppler sampling. We did not find it essential to the proper recognition of anatomically abnormal hearts.