A 17-year-old, nationally ranked, male tennis player (AH) had been experiencing heat cramps during tennis match play. His medical history and previous physical exams were unremarkable, and his in-office blood chemistry profiles were normal. On-court evaluation and an analysis of a 3-day dietary record revealed that AH's sweat rate was extensive (2.5 L.hr-1) and that his potential daily on-court sweat sodium losses (89.8 mmol.hr of play-1) could readily exceed his average daily intake of sodium (87.0-174.0 mmol.day-1). The combined effects of excessive and repeated fluid and sodium losses likely predisposed AH to heat cramps during play. AH was ultimately able to eliminate heat cramps during competition and training by increasing his daily dietary intake of sodium.