Objective: This study was conducted to determine whether the incidence of ectopic pregnancy follows a seasonal rhythm of occurrence. Study Design: A retrospective analysis was performed on 15,639 pregnancies reported in a single department in 5.5 years (January 1992-June 1997). For each pregnancy the time of conception was estimated from medical records. Frequencies of conceptions that terminated in birth, spontaneous abortion, legally induced abortion, and ectopic pregnancy were distributed across the 12-month period. Results: Total pregnancy showed a seasonal rhythm of occurrence, with peak values in March. The rate of ectopic pregnancy showed a rhythm with 2 peaks, in June and December. Nadirs were coincident with the peak and nadir of total conceptions and also with the equinoxes. Spontaneous abortions tended to show a rhythm the inverse of that of ectopic pregnancy. Conclusions: These data show an influence of the seasons on the rate of ectopic pregnancy, which may have implications for both the understanding of ectopic pregnancy’s causative mechanisms and its prevention. (Am J Obstet Gynecol 1999;180:1067-71.)