A knowledge of the brain-blood partition coefficient (lambda) for water is usually required for the measurement of CBF with [15O]water. The currently accepted value for whole-brain lambda, 0.95-0.96 ml/g, calculated from brain and blood water content data, is incorrect because in the calculation, the blood water content was not adjusted for the density of blood. The correct value is 0.90 ml/g. Variations in brain or blood water content affect lambda. Thus, lambda changes during development of the brain and varies regionally in it, even among different gray matter structures, owing to variation in brain water content. In addition, lambda would be expected to vary with the hematocrit, owing to changes in blood water content. The impact of using an incorrect value for lambda will depend on the sensitivity of the CBF measurement technique used to errors in lambda.