Patients with cancer often have disturbances of cellular immunity. Changes of cellular immunity are frequently associated with tumor progression or an increased risk of a relapse. Cellular immunity is studied with an in vivo test system using 7 different recall antigens. We studied the cutaneous reaction of 80 children with various malignant diseases at time of diagnosis, at the end of the chemotherapy or prior to a relapse of the disease using the multitest Merieux. Before starting treatment patients with systemic diseases showed only a little response to the recall antigens. Patients with solid tumors had normal responses. No prognostic meaning of the multitest results were seen, if it was done before starting the treatment. At time of a relapse, these patients mostly had negative skin reactions using the multitest. It has to be studied if these results show a new systemic disease or if they are expression of an ongoing disturbance of cellular immunity of these patients.