Objectives: The association between tibial morphology and tibial stress fractures or tibial stress syndrome was examined in triathletes with an unusually high incidence of these injuries. Design: A cross-sectional study design examined associations between tibial geometry from MRI images and training and injury data between male and female triathletes and between stress fracture (SF) and non-stress fracture (NSF) groups. Methods: Fifteen athletes (7 females, 8 males) aged 17-23 years who were currently able to train and race were recruited from the New Zealand Triathlete Elite Development Squad. Geometric measurements were taken at 5 zones along the tibia using MRI and compared between symptomatic and asymptomatic tibiae subjects. Results: SF tibiae displayed either oedema within the cancellous bone and/or stress fracture on MRI. When collapsed across levels, symptomatic tibiae had thicker medial cortices (F1,140 = 9.285, p = 0.003), thicker lateral cortices (F1,140 = 10.129, p = 0.002) and thinner anterior cortices (F1,140 = 14.517, p = 0.000) than NSF tibiae. Only medial cortex thickness in SF tibia was significantly different (F4,140 = 3.358, p = 0.012) at different levels. Follow-up analysis showed that athletes showing oedema within the cancellous bone and/or stress fracture on MRI had, within 2 years of analysis, subsequently taken time off training and racing due a tibial stress fracture. Conclusions: The thinner anterior cortex in SF tibiae is associated with a stress reaction in these triathletes.