Increased levels of mucin-like carcinoma-associated antigen (MCA) in breast cancer patients with no evidence of disease following the treatment of the primary disease created a dilemma of 'to treat' or 'wait and see'. One might assume that early treatment of clinically undetectable disease on the basis of an elevated serum level of a sensitive and reliable tumor marker, may improve the treatment results, and even prolong the patient's survival. 'Wait and see' on acceptance of the notion that even early metastatic disease, still manifested only by uprising MCA levels, is incurable, and treatment should be kept in reserve for palliation of symptomatic disease. Sixty-one breast cancer patients with increasing MCA levels but without evidence of metastatic disease were randomized for tamoxifen 20 mg b.i.d. or to follow-up till relapse. The results for a median follow-up period of one year were encouraging. The non-treated patients experienced a significantly higher relapse rate (24.1%) than the tamoxifen-treated subjects (0%; p=0.012). The results for a median follow-up of 5 years were disappointing. The overall relapse rate was 22.2%. The relapse rate among the control patients was 25.8% while in the treatment arm it was 17.4% (p=0.46). The event-free survival and the pattern of relapse were similar in both arms. Tamoxifen may therefore be reserved for overt metastases, and not wasted on asymptomatic subclinical disease. It seems that there is no yield in terms of event-free survival for MCA measurements in breast cancer patients during the 5-year follow-up period.

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