In children with cancer a reduced neutral urine DNase activity was observed at the end of cytostatic therapy. It is presumed that disorders in the kidney functions are the cause. A low neutral DNase activity is found in the blood; the activity is inhibited by inhibitors. The inhibitors of the enzyme are split away in the kidneys so that the neutral DNase activity in urine increases. Many cytostatic drugs can induce damage to the kidneys. This may be the reason that not all inhibitors of neutral DNase are split away in the kidneys and the activity of the neutral urine DNase is reduced at the end of cytostatic therapy. The process of removing the inhibitors is independent of the filtering capacity of the kidneys.