Cancer is the second leading cause of death in developed countries, making it a global public health problem. In this scenario, early detection is the key to successful treatment. Tissue biopsy, the current gold standard for cancer diagnosis, offers reliable results, but it is feasible only when the mass becomes detectable. On the other hand liquid biopsy, a promising experimental system, not yet implemented within clinical practice, allows early detection as its functioning relies on the analysis of body fluids. Yet, its results are less reliable if compared to those of tissue biopsy as, for instance, false positives and false negatives might occur. Despite technical features, the tradeoff between a reliable diagnosis available at a later time and a potentially less reliable diagnosis available at an early stage poses significant ethical challenges in the clinical scenario which involve, among other aspects, informed consent, communication, and patient-physician encounter.