Purpose: FOGT has initiated two prospective controlled randomized trials to improve adjuvant therapy of colon cancer stages UICC II / T4N0M0 and III (FOGT1) and rectal cancer stages UICC II and III (FOGT2). This interim report analyzes the toxicity and acceptance of the 3 treatment arms. Patients and Methods: ‘Standard group’ (arm A) is treated with 5-fluorouracil (5FU) plus levamisole (LEV, Ergamisol®). In arm B 5FU is modulated by folinic acid (FA, Rescuvolin®) (5FU + FA + LEV), in arm C by interferon alpha (IFNα, Roferon®) (5FU +IFNα + LEV). Rectal cancers, in addition, are irradiated with 50.4 Gy. Chemotherapy doses are adjusted to toxicity if toxic events > WH02 occur. Until October 1996, 56 hospitals recruited a total of 946 patients (FOGT1 521, FOGT2 425 patients). Toxicity and discontinuance rates were noted. Results: Among the 839 patients evaluable according to ‘intention to treat’ (FOGT1: 464, FOGT2: 375 patients), a toxic event > WH02 occurred in 139 patients (17%). Toxicities > WH02 in arms A, B, and C of FOGT1 were 5, 7, and 21%, respectively, whereas the respective values in FOGT2 were 20, 16, and 38%. Discontinuance rates in arms A, B, and C of FOGT1 were 23, 17, and 25%, respectively, while the respective rates in FOGT2 were found to be 20, 20, and 23%. Treatment was stopped in arms A, B, and C of FOGT1 either because of toxicity or on patient’s demand in 11, 8, and 16%. In FOGT2, treatment in arms A, B, and C was terminated for these reasons in 7, 13, and 10% of the cases, respectively. The overall discontinuance rate due to toxicity or patient’s demand was 11%. Toxicity in arm C seemed to be higher than in the other treatment arms and was mainly due to leukopenia and diarrhea. One toxic death was noted in FOGT1 arm A, probably due to 5FU cardiac side effects. Conclusion: The rate of discontinuance is within those of other trials, e.g. the Inter-group Studies (Moertel 1990), so that FOGT1 and FOGT2 trials are safe and acceptable concerning toxicity and patient compatibility.