Midstream urine samples were examined by phase-contrast microscopy before and immediately after 48 subjects participated in a long-distance run. Minor abnormalities were found in six samples before exercise. Eighteen subjects developed proteinuria and five haematuria on dipstick testing after exercise. Forty-four subjects had increased urinary red-cell counts after exercise; of these, 33 had counts above the normal range (8000/ml). In all subjects urinary red cels were dysmorphic both before and after exercise, indicating a glomerular source. Ten subjects developed red-cell casts and 42 showed an increase in hyaline and hyaline-granular casts after exercise. There were modest increases in urinary white-cell counts in 35 subjects but little change in urine pH or osmolality with exercise. This study confirms that urinary red-cell counts commonly increase appreciably after exercise. The dysmorphic appearance of the red cells together with the presence of red-cell casts indicates a glomerular source for this common form of exercise haematuria.