Current hydration guidelines are designed to address the fine balance between minimising dehydration while reducing the risk of hyponatremia. During prolonged cycling events small discrepancies between drinking behaviour and fluid requirements may be detrimental to health and performance. The present study aimed to investigate the hydration practices of competitors in a 387 km cycle race in order to evaluate the effect on fluid balance and monitor the prevalence of dysnatremia. Eighteen participants provided blood and urine samples pre- and post-race to measure sodium and fluid balance. Sweat samples were collected via patches for analysis of sodium concentration. Body weight was measured at the start and end of the race. On average participants consumed 0.58 L h(-1) of fluid. Upon completion of the race 7 of the 18 (39%) cyclists had blood sodium concentrations of 135 mmol L(-1) or lower with one cyclist recording a value of 132 mmol L(-1). Only two cyclists appeared to be moderately dehydrated. A post-race questionnaire indicated cyclists were most concerned with preventing dehydration. It appears that cyclists taking part in prolonged endurance events are at more risk of hyponatremia than dehydration and do not readily change their drinking behaviour to match their sweat losses.