Objective: In contrast to tubal abortions, viable ectopic pregnancies in color Doppler ultrasonography exhibit a signal-intensive ring around the gestational sac. We investigated the underlying differences in implantation and placentation. Study Design: Histologic sections of fallopian tubes carrying viable tubal pregnancies (13 patients) and tubal pregnancies that aborted (8 patients) were immunostained for cytokeratin, MIB-1, CD-34, and CD-68. The data were studied by computer-aided image analysis followed by statistical evaluation (Student t test, P < .05). Results: In contrast to tubal abortions, viable tubal pregnancies are characterized by implantation at the mesosalpingial rather than at the antimesosalpingial side of the organ. They exhibit deeper trophoblast invasion into the thickened tubal wall, more intense trophoblast proliferation (P < .001), and increased villous vascularization (P < .001). Conclusion: The morphologic findings correlate with preoperative Doppler ultrasonography. They suggest that trophoblast invasion, placental growth, and the fate of tubal pregnancies depend on the implantation site. They encourage a conservative management of anti–mesosalpingially implanted, nonviable ectopic pregnancies in clinically stable patients. (Am J Obstet Gynecol 1999;181:615-20.)