Purpose: FOGT has initiated two prospective controlled randomized trials to improve adjuvant therapy of colon cancer stage UICC II/T 4NOM0 and III (FOGT-1) and rectal cancer stage UICC II and III (FOGT-2). This interim report analyses toxicity and acceptance of the three treatment arms.Methods: Standard group (ArmA) consists of 5-FU+levamisole, (LEV, Ergamisol R ). In ArmB 5-FU is modulated by Folinic Acid (FA, Rescuvolin R ) (5-FU+FA+LEV), in ArmC with Interferon alpha (IFNa, Roferon R ) (5-FU+IFNa+LEV). Rectal cancers, in addition, were irradiated with 54 Gy. Chemotherapy doses are adjusted to toxicity, if toxic events > WHO 2 occur. Until 10/96, 56 hospitals have recruited a total of 946 pts. (FOGT 1 521 pts., FOGT 2 425 pts.). Toxicity and discontinuance rates were noted.Results: Among the 839 pts. evaluable according to intention to treat (FOGT-1: 464, FOGT-2: 375), a toxic event > WHO2 occurred in 139 (17%), and treatment was stopped in 177 (21%). Toxicities > WHO2 in FOGT-1 A, B, C were 5%, 7%, 21%, in FOGT-2 20%, 16%, 38%, respectively. Discontinuance rates in Arms A, B, and C of FOGT-1 were 23%, 17%, 25%, and 20%, 20%, 23% in FOGT-2, respectively. Treatment was stopped in Arms A, B, C because of toxicity or patient's demand in 11%, 8%, 16% in FOGT-1 or 7%, 13%, 10% in FOGT-2, respectively. The overall discontinuance rate due to toxicity or patients' demand was 11%. Toxicity in ArmC seemed to be higher, and was mainly due to leukopenia and diarrhea.Conclusion: The rate of discontinuance is within those of other trials, e.g. the Intergroup Studies (Laurie 1990, Moertel 1990), so that FOGT-1 and FOGT-2 trials are safe and acceptable concerning toxicity and patient compatibility.