Neopterine is considered to be one of the markers of immunity system activity. An increased level of this pteridine derivate is determined in blood or urine in infections, transplant rejections, and in other conditions accompanied by changes in the immunity system. Monitoring its level in various body liquids can be important when assessing the immunity system condition, nevertheless, its non-specificity causes difficulties when interpreting the results. Up to now, very few papers have been published dealing with the determination of neopterine levels in patients with IBD. We studied a group of 25 patients who are monitored in a longterm manner at our clinic suffering from Crohn's disease. During 1993 and 1994, we were determining their levels of neopterine in blood together with other parameters. It was a group of patients chosen at random in different stages of disease, and we were interested in the correlation of the height of neopterine level with the activity of Crohn's disease. We proved statistically unimportant correlation between the height of neopterine level in blood serum and Crohn's disease activity. The question is discussed whether the examination of this marker be useful when monitoring Crohn's disease activities.