Functional constipation (FC) often begins in the first year of life. Although standard definitions and criteria have been formulated to describe FC, these are rarely used in research and clinical practice. The aim of the study is to systematically assess how definitions and outcome measures are defined in therapeutic randomized controlled trials (RCTs) of infants with FC. PubMed, EMBASE, and Cochrane databases were searched. Studies were included if it was a (systematic review of) therapeutic RCT, children ≤4 years old, they had FC, a clear definition of constipation was provided, and were written in English. Quality was assessed using the Delphi list. A total of 1115 articles were found; only 5 studies fulfilled the inclusion criteria. Four different definitions were used, of which only 2 used the internationally accepted Rome III criteria. Defecation frequency was used as primary outcome in all included trials and stool consistency in 3 trials. Two trials involving infants investigated new infant formulas, whereas the third RCT evaluated the efficacy of a probiotic strain. The 2 trials including infants up to 4 years of age compared polyethylene glycol without electrolytes (PEG4000) with lactulose and milk of magnesia. All of the trials used nonvalidated parental diaries. Different definitions and outcome measures for FC in infants are used in RCTs. Disappointingly, there is a lack of well-designed therapeutic trials in infants with constipation. To make comparison between future trials possible, standard definitions, core outcomes, and validated instruments are needed.