69 children aged from shortly after birth to 12 years were investigated to assess the effect of frusemide on the changes in hematocrit and serum osmolality produced by an angiocardiographic contrast medium during cardiac investigation. Frusemide significantly accelerated the return to control values of these two parameters and led to enhanced urinary excretion of the contrast medium. The effect was already noticeable 10 min after contrast medium injection, but because ofindividual variation it should be monitored by repeated hematocrit estimations.