Research has shown that patients' preferences for adjuvant chemotherapy do not change as a result of experience. However, the preferences of experienced patients are usually more favourable than those of inexperienced patients. These results indicate a shift in preferences after the decision to proceed with adjuvant chemotherapy has been made, but before actual experience. We tested this assumption in early-stage breast and colorectal cancer patients. We asked patients to provide their preferences for chemotherapy before surgery and thus before they knew whether chemotherapy would be advised (T(1)), after surgery but before the start of chemotherapy (T(2)) and about 1 month after chemotherapy (T(3)). Patients who did not undergo chemotherapy co-operated at similar points in time. Preferences were measured on a nine-point scale, ranging from (1) 'very strong preference for no chemotherapy' to (9) 'very strong preference for chemotherapy'. As hypothesized, the preferences of patients who would be treated with chemotherapy became more favourable after the treatment decision had been made (n = 7, P = 0.06). The preferences of patients for whom chemotherapy was not part of the treatment plan showed the opposite effect (n = 38, P = 0.03). We did not find any effect of experiencing treatment (n = 22, P = 0.62) or the passing of time (n = 81, P = 0.25) on the stability of preferences. We conclude that the frequently observed discrepancy in treatment preferences between experienced and inexperienced patients seems to be an effect of the treatment decision and not of experience of the treatment.