OBJECTIVE: To compare the clinical performance of two glass-ionomer cements (GICs)for Class II restorations in primary molars: a conventional cement (Fuji II) and a resin-modified cement (Vitremer). DESIGN: Split mouth and random assignment to thetwo materials were used for the majority of the molars. SAMPLE AND METHODS: Forty consecutive 4-7-year-old children were included. One operator made 115 restorations: 53 with Vitremer and 62 with Fuji II. The restorations were evaluated clinically, radiographically and from colour photographs. RESULTS: The cumulative success rate of the Vitremer restorations was 94% and that of the Fuji II restorations 81%. The difference is statistically significant. The risk of a failed restoration was more than five times higher with Fuji II than with Vitremer as the restorative material. Of the 13 unsuccessful restorations, seven had lost their retention, four had secondary caries, and two were fractured. CONCLUSIONS: The resin-modified GIC offered advantages over the conventional GIC for restoring approximal caries in primary molars.