This research explores the interplay between students' understandings of proportional and functional relationships. Approximately 90 students participated in an early algebra intervention in Grades 3- 5. Before the intervention and after each year of the intervention, we evaluated their understandings of proportional and functional relationships. Data revealed that among Grades 4 and 5 students who identified a correct function rule, a higher percentage were unsuccessful solving a proportional reasoning problem than those who were not able to identify a correct function rule. Namely, the data suggest that students' development of functional thinking may interfere with their development of understanding proportional relationships. [For complete proceedings, see ED581294.]