Background: Immunoglobulin κ and λ free light chains (FLC) are important serum biomarkers for diagnosing and monitoring plasma cell dyscrasias (via the κ:λ FLC ratio), and assessing immune competence and activation status (via ∑FLC). FLCs are produced, in excess of heavy chains, from healthy plasma cells during immunoglobulin production, but unlike intact immunoglobulins that are cleared by cellular catabolism over a number of weeks, FLC are rapidly cleared from the bloodstream by the renal glomerulus with a half-life of 3 (κ FLC)-6 (λ FLC) hours. Marathon running has been shown to acutely and transiently decrease renal function, however, the impact of prolonged aerobic exercise on FLC levels remains unknown. Methods: We measured serum FLC levels in 60 runners before, and immediately after, the 2010 Eindhoven Marathon. Results: A significant increase (p < 0.01) in κ FLC levels was observed after the marathon, and κ FLC correlated positively with serum creatinine levels. No changes were observed for λ FLC, and thus, there were subtle elevations