Data sourcesPubMed, Scopus, Web of Science, Virtual Health Library (VHL), Cochrane Library, Clinical Trials and OpenGrey.Study selectionRandomised controlled trials comparing the clinical effectiveness of Class II restorations performed with conventional (C-GIC) or resin-modified glass ionomer cement (RM-GIC) and composite resin (CR) in primary molar teeth. No date of publication or language restrictions.Data extraction and synthesisStudy selection was carried out independently by two reviewers, with abstracted data and risk of bias assessment being performed using the Cochrane tool. Data on the restorations were dichotomised as acceptable' (restorations without need of replacement or repair) or 'unacceptable' (restorations presenting failures or requiring repair or replacement) after which a number of meta-analyses were conducted.ResultsTen studies were included in qualitative synthesis, and nine contributing to the meta-analyses. Six studies used a split-mouth design and four a parallel design. Seven studies used USPHS criteria, two applied the FDI criteria and one used their own. Seven studies reported restorations were placed under rubber dam isolation with the other three using cotton roll isolation. Six studies were at low risk of bias and four unclear risk of bias. GIC and CR presented similar failure patterns (Risk Difference [RD] = -0.04 (95%CI; -0.11 to 0.03) p=0.25, I2 = 51%), irrespective of follow-up period, type of GIC used, method of isolation or criteria used for assessment. GICs exhibited significantly lower values of secondary carious lesions ([RD] = 0.06 (95%CI; 0.0 to 0.10), p=0.008, I2 = 0%).ConclusionsGICs and CRs have comparable clinical performance in Class II restorations in primary molars. GICs did show superior performance in the occurrence of secondary carious lesions, especially when RM-GIC under rubber dam isolation was used.