In 93 patients with surgically proved ectopic gestation, the initial serum .beta.-human chorionic gonadotropin (hCG) concentration was 2.0-5260 ng/ml; in 33 (35%) the values were < 100 ng/ml. Of 25 patients with serial hCG data, 20 showed a plateau or fall in serum concentrations over at least 48 h, thereby identifying the gestation as nonviable. A total of 64 patients underwent sonographic evaluation; 35 of these also underwent at least 1 quantitated hCG determination. Correlating a single hCG value with the sonographic diagnosis correctly identified the gestation as nonviable in 21% of the cases, but a definite diagnosis was not possible in the rest. For the clinically stable patient with a suspected ectopic pregnancy, serial hCG determinations offer a useful technique for evaluating gestational viability.