In the course of a 19-d study of renal function in five ultramarathon runners, before, during and after a 90 km race, one runner developed transient oliguria with renal tubular dysfunction and anuria during and immediately after the race. Other features of the renal failure were an 84-fold increase in urine beta 2-microglobulin excretion (from 0.19 to 16.0 micrograms.min-1) and a much smaller increase in urine total protein excretion (from 0.07 to 0.18 mg.min-1) during the post-race period. Post-race creatinine clearance remained below pre-race levels throughout the study, varying between 42.8 and 72.9 ml.min-1, in contrast to the post-race 49% increase in the remaining runners (from 138.1 +/- 12.9 to 205.5 +/- 59.9 ml.min-1). Osmolal clearance also remained low (0.31 to 0.98 ml.min-1) compared with the pre-race values (1.46 +/- 0.02 ml.min-1), as did the urine flow rates (0.11 to 0.18 ml.min-1) compared with the pre-race values (0.34 +/- 0.02 ml.min-1). This renal dysfunction persisted despite the patient receiving 2 l of intravenous fluids immediately after the race and probably resulted from fluid restriction during the race. There was full recovery of renal function 1 yr later when the subject again ran the Comrades Marathon.