Purpose: To investigate the probability of survival of different implant-abutment connection designs in narrow versus standard-diameter implants supporting anterior crowns. Materials and Methods: A total of 108 implants of either 3.5-mm or 4.0-mm diameter (narrow and standard, respectively) (10 mm in length, Implacil de Bortoli) were divided into six groups (n = 18 each) as follows: external hexagon 3.5 mm or 4.0 mm (EH3.5 or EH4.0), internal hexagon 3.5 mm or 4.0 mm (IH3.5 or IH4.0), and Morse taper 3.5 mm or 4.0 mm (MT3.5 or MT4.0). The corresponding abutments were screwed to the implants, and standardized maxillary central incisor metal crowns were cemented and subjected to step-stress accelerated life testing in water. Use-level probability Weibull curves and reliability for a mission of 50,000 and 100,000 cycles at 100 N and 150 N (90% 2-sided confidence intervals [CI]) were calculated. Polarized-light and scanning electron microscopes were used to assess the failure modes. Results: The calculated reliability with 90% CI for a mission of 50,000 cycles at 100 N and 150 N showed that cumulative damage from the respective loads would lead to ~93% and ~18% implant-supported restoration survival in group EH3.5, ~99% and ~1% in group IH3.5, ~97% and ~89% in the MT3.5, ~100% and ~99% in the group EH4.0, ~100% and ~100% in group IH4.0, and ~99% and ~99% in group MT4.0. For the 100,000-cycle mission, the probability of survival estimated at 100 N and 150 N was, respectively: 0% for EH3.5 and IH3.5 at both load levels, ~96% and ~87% for the MT3.5, 100% and ~99% for EH4.0, 100% and ~99% for IH4.0, and 98% and ~92% for the MT4.0. Conclusion: A significant decrease in the probability of survival as a function of elapsed fatigue cycles and load increase was observed for narrow implants only with EH and IH implant-abutment connections, but not on Morse taper.