Age-related macular degeneration (AMD) is a leading cause of vision loss, with a strong genetic susceptibility at the complement factor H (CFH) locus. This locus encodes a series of complement regulators: factor H (FH), a splice variant factor H-like 1 (FHL-1), and five factor H-related proteins (FHR-1 to FHR-5), all involved in the regulation of complement factor C3b turnover. Little is known about the influence of AMD associated variants at this locus on FHL- 1 and FHR protein levels. We have used a bespoke targeted mass spectrometry assay to measure the circulating levels of all seven complement regulators, and demonstrated elevated levels in 352 advanced AMD individuals for all FHR proteins (FHR-1, P=2.4x10-10; FHR-2, P=6.0x10-10; FHR-3, P=1.5x10-5; FHR-4, P=1.3x10-3; FHR-5, P=1.9x10-4) and FHL-1 (P=4.9x10-4) compared to 252 controls, while no difference was seen for FH (P=0.94). Genome-wide association analyses in controls revealed genome-wide significant signals at the CFH locus for all five FHR proteins, and univariate Mendelian randomization analyses strongly supported the association of FHR-1, FHR-2, FHR-4 and FHR-5 with AMD susceptibility. These findings provide a strong biochemical explanation for how geneticallydriven alterations in circulating FHR proteins could be major drivers of AMD and highlight the need for research into FHR protein modulation as a viable therapeutic avenue for AMD.