The present study investigated the changes in performance and sex difference in top performers for all ultra-triathlon distances held between 1978 and 2013 from the Ironman (i.e. 3.8km swimming, 180km cycling and 42km running) to Double Deca Iron ultra-triathlon distance (i.e. 76km swimming, 3,600km cycling and 840km running). The fastest men ever were faster than the fastest women ever for split disciplines and overall race times with the exception for swimming in Quintuple Iron ultra-triathlon (i.e. 19km swimming, 900km cycling and 210.1km running). The correlation analysis showed an increase in the sex difference with increasing length of the race distance for swimming (r2=0.67, P=0.023), running (r2=0.77, P=0.009), and overall race time (r2=0.77, P=0.0087), but not for cycling... (r2=0.26, P=0.23). For the annual top performers, split times decreased across years non-linearly in female and male Ironman triathletes in swimming, cycling, running and overall race times. For longer distances, cycling split times decreased linearly in male Triple Iron ultra-triathletes and running split times decreased linearly in male Double Iron ultra-triathletes but increased linearly in female Triple and Quintuple Iron ultra-triathletes. Overall race times increased non-linearly in female Triple and male Quintuple Iron ultra-triathletes. The sex difference decreased non-linearly in swimming, running, and overall race time in Ironman triathletes, but increased linearly in cycling and running and non-linearly in overall race time in Triple Iron ultra-triathletes. These findings suggest that women reduced the sex difference non-linearly in shorter ultra-triathlon distances (i.e. Ironman) but for longer distances than the Ironman, the sex difference increased or remained unchanged across years. It seems very unlikely that female top performers will ever outrun male top performers in ultra-triathlons. Furthermore, the non-linear change in speed and sex difference in Ironman triathlon suggests that Ironman triathletes have reached their limits in performance.