Fifty-four pairs of restorations (one glass ionomer and the other amalgam) were placed in the mouths of 36 xerostomic head and neck cancer patients. Patients were either fluoride users or fluoride nonusers. In patients who used a daily application of a mildly acidic (pH 5.8) sodium fluoride gel, glass-ionomer cements failed and amalgam restorations did not (P < 0.0001). In patients who neglected to use their topical fluoride as directed, glass-ionomer cement restorations did not fail, but amalgam restorations did (P < 0.001). The mean time to restoration loss for both restorative materials was 8.5 months. In severely xerostomic patients these findings were exaggerated.