The Hall Technique (HT) is a method for managing carious primary molars. Decay is sealed under pre-formed metal crowns without any caries removal, tooth preparation, or local anesthesia. The aim of this study was to compare HT clinical/radiographic failure rates with General Dental Practitioners' (GDPs) standard (control) restorations. We conducted a split-mouth, randomized control trial (132 children, aged 3-10 yrs, GDPs n = 17) in Scotland. There were 264 study teeth with initial lesions, 42% of which were radiographically > half-way into dentin, and 67% of which had Class II restorations. Teeth were randomized to HT (intervention) or GDPs' usual treatment (control). Annual clinical/radiographic follow-up data were recorded. Ninety-one patients (69%) had 48 months' minimum follow-up. At 60 months, 'Major' failures (irreversible pulpitis, loss of vitality, abscess, or unrestorable tooth) were recorded: HT, 3 (3%); control restorations, 15 (16.5%) (p = 0.000488; NNT 8); and 'Minor' failures (reversible pulpitis, restoration loss/wear/fracture; or secondary caries): HT, 4 (5%); control restorations, 38 (42%) (p < 0.000001; NNT 3). Overall, there were follow-up data for 130 patients (2-60 mos): 'Major' failures: HT, 3 (2%); control restorations, 22 (17%) (p = 0.000004; NNT 7); and 'Minor' failures, HT, 7 (5%); control restorations, 60 (46%) (p < 0.000001; NNT 3). Sealing in caries by the Hall Technique statistically, and clinically, significantly outperformed GDPs' standard restorations in the long term (Trial registration no. ISRCTN 47267892).