A large proportion of children younger than five years of age in high-income countries experience significant non-parental care. Centre-based day care services may influence the development of children and the economic situation of parents. To assess the effects of centre-based day care without additional interventions (e.g. psychological or medical services, parent training) on the development and well-being of children and families in high-income countries (as defined by the World Bank 2011). In April 2014, we searched CENTRAL, Ovid MEDLINE, EMBASE, PsycINFO, the Education Resources Information Center (ERIC) and eight other databases. We also searched two trials registers and the reference lists of relevant studies. We included randomised and quasi-randomised controlled trials of centre-based day care for children younger than five years of age. We excluded studies that involved co-interventions not directed toward children (e.g. parent programmes, home visits, teacher training). We included the following outcomes: child cognitive development (primary outcome), child psychosocial development, maternal and family outcomes and child long-term outcomes. Two review authors independently assessed the risk of bias and extracted data from the single included study. We contacted investigators to obtain missing information. We included in the review one trial, involving 120 families and 143 children. Risk of bias was high because of contamination between groups, as 63% of control group participants accessed day care services separate from those offered within the intervention. No evidence suggested that centre-based day care, rather than no treatment (care at home), improved or worsened children's cognitive ability (Griffiths Mental Development Scale, standardised mean difference (SMD) 0.34, 95% confidence interval (CI) -0.01 to 0.69, 127 participants, 1 study, very low-quality evidence) or psychosocial development (parental report of abnormal development, risk ratio (RR) 1.21, 95% CI 0.25 to 5.78, 137 participants, 1 study, very low-quality evidence). No other measures of child intellectual or psychosocial development were reported in the included study. Moreover, no evidence indicated that centre-based day care, rather than no treatment (care at home), improved or worsened employment of parents, as measured by the number of mothers in full-time or part-time employment (RR 1.12, 95% CI 0.85 to 1.48, 114 participants, 1 study, very low-quality evidence) and maternal hours per week in paid employment (SMD 0.20, 95% -0.15 to 0.55, 127 participants, 1 study, very low-quality evidence) or household income above £200 per week (RR 0.86, 95% CI 0.57 to 1.29, 113 participants, 1 study, very low-quality evidence). This study did not report on long-term outcomes for children (high-school completion or income). This review includes one trial that provides inconclusive evidence as regards the effects of centre-based day care for children younger than five years of age and their families in high-income countries. Robust guidance for parents, policymakers and other stakeholders on the effects of day care cannot currently be offered on the basis of evidence from randomised controlled trials. Some trials included co-interventions that are unlikely to be found in normal day care centres. Effectiveness studies of centre-based day care without these co-interventions are few, and the need for such studies is significant. Comparisons might include home visits or alternative day care arrangements that provide special attention to children from low-income families while exploring possible mechanisms of effect.