Caries incidence is falling in many developed countries but is believed to be increasing in developing countries. Insofar as treatment for caries is provided, ordinary amalgam restorations are usually the standard choice. These carry a risk of failing due to secondary caries. The aim of this study was to compare the occurrence of secondary caries adjacent to fluoride-containing amalgam with that adjacent to a conventional amalgam under field conditions in Bahrain. Children (n = 415) aged 6-14 yr who required occlusal restorations in two homologous contralateral permanent molar teeth were identified. Using the split-mouth design, one tooth received a fluoride-containing amalgam filling and the other a conventional amalgam restoration. A random method was used to determine which type of filling was placed on each side. After 4 yr, 357 children were still available for examination; of these 5 had to be eliminated from analysis for various reasons. Of the 704 teeth in the analysis, secondary caries had occurred in 135 (86 conventional amalgam, 49 fluoridated amalgam). By comparison with the conventional amalgam, the effectiveness of the fluoride amalgam was 43% (95% C.I. 24.4%, 57.1%) and the net gain 10.3% (95% C.I. 4.75%, 16.3%). The relative risk was 0.570 (95% C.I. 0.444%, 0.731%). Since fluoride amalgam has identical handling properties to conventional amalgam, performs similarly under clinical conditions and costs about the same, it should be the amalgam of choice for restorations in communities where the incidence of secondary caries is high.