The strengthening of steel structures using adhesively bonded carbon fibre-reinforced polymers (CFRP) has emerged as a promising technique to enhance fatigue resistance, particularly in aging infrastructure. However, the wide variation in experimental setups, material systems, and application strategies has limited the comparability of available data. This study presents a meta-analysis of approximately 250 fatigue tests on centrally notched steel specimens strengthened with CFRP under constant amplitude loading. By systematically harmonizing the data, five key parameters governing the effectiveness of CFRP strengthening are identified: application type (single- or double-sided), relative positioning to the fatigue crack, pre-damage level of the steel, stiffness ratio between CFRP and steel, and degree of prestress. The analysis shows that fatigue life can be significantly increased through optimized combinations of these parameters, with double-sided application and full crack coverage proving particularly effective. The results highlight both the potential and limitations of CFRP-based fatigue strengthening and provide a foundation for the development of a structured research database. This enables improved comparability of future studies and supports the practical implementation of CFRP systems in fatigue-critical steel components, such as those found in bridges and other civil infrastructure.