In 2011, the Committee on Nutrition of the European Society for Paediatric Gastroenterology, Hepatology, and Nutrition systematically reviewed published evidence on the safety and health effects of the administration of formulae supplemented with probiotics and/or prebiotics compared with unsupplemented formulae. The document could serve as an example of problems relating to the choice and definition of outcomes assessing the addition of new ingredients to infant formulae. The studies were often too small with insufficient power to identify relevant effects, and the follow-up periods in the trials were too short. The clinical outcomes, even those relating to the same domain (eg, gastrointestinal infections) differed. Even if the same outcomes were measured, the definitions of the outcomes were heterogenous, often not widely agreed upon, or just lacking. The use of inappropriate outcome measures and/or their definitions may result in misleading conclusions. It may also lead to an overestimation or underestimation of potential benefits of the intervention or fail to reveal any potential benefits. There is a need for well-designed and carefully conducted randomized controlled trials, with relevant inclusion/exclusion criteria and adequate sample sizes. These studies should use validated clinical outcome measures.