Randomized controlled trials are the gold standard for determining the efficacy of therapeutic interventions. However, medical practice has not evolved around the concept of randomized trials, but around the idea of careful observations, (anecdotal) case studies and the evaluation of retrospective data. Interventions discovered by these means and taken forward into clinical practice became standard practice as they continued to be superior when compared with prior or alternative types of treatment. Personalized medicine refers to an approach of clinical practice where a particular treatment is not chosen based on the 'average patient', but on characteristics of an individual patient, for example, a genetic profile that may vary from one patient to another, and therefore, allows to 'personalize' the treatment to a patient's individual needs. While the call for prospective randomized controlled trials to assess the effective use of such measurement may make sense in some cases, it is, when applied without distinction, hindering the implementation of personalized medicine. Important evidence for the validity and clinical effectiveness of using biomarkers, for example, a patient's genetic profile, can be gained from alternative approaches, including case-control and cohort studies, and retrospective analyses of data. Hence, we need to re-focus on approaches that are neither new nor unproven, but have been ignored over the last few decades.