Rhabdomyolysis, secondary to exertion is known to result in myoglobinuria and is occasionally associated with acute renal failure. In this study myoglobinaemia occurred in 25 of 44 runners completing a 99 km marathon. A marked rise in the values of myoglobin, lactate and the enzymes creatine kinase (CPK), aspartate transaminase (AST) and lactic dehydrogenase (LDH) was noted. A linear correlation was demonstrated between the level of serum myoglobin and the serum concentrations of urate, CPK, AST and LDH. Both the myoglobin itself and the increased concentration of urate may contribute to the acute renal failure. The pathophysiology of rhabdomyolysis during exertion is discussed in the context of other causes of myoglobinuria. A classification of rhabdomyolysis and myoglobinuria is suggested.