The aim of this study was to evaluate flossing as a diagnostic method for interproximal gingival bleeding in children. For this crossover study, 23 pre-schoolchildren presenting neither restorations nor approximal carious cavities and with at least 15 percent of gingival bleeding sites were selected. Examinations were performed at three different moments (3-4 days interval). Examinations comprised repeated measurements of two gingival indices with a 10-minute interval in the following sequences: the Ainamo & Bay Gingival Bleeding Index (GBI) followed by the Carter & Barnes flossing index (CBI); CBI followed by GBI; and GBI followed by GBI. Data analysis was performed only for the interproximal sites, considering the GBI as the gold-standard. Agreement between indices, sensitivity (SE), specificity (SP), positive (PPV) and negative predictive values (NPV) were estimated. Percentage agreements in sequences GBI-CBI, CBI-GBI and GBI-GBI were 70.3 percent, 76.4 percent and 84.5 percent, respectively. Validation of flossing in the first sequence (GBI-CBI) resulted in values of 0.61 (95 percentCI 0.53 - 0.68), 0.72 (95 percentCI 0.69 - 0.76), 0.33 (95 percentCI 0.28 - 0.39) and 0.89 (95 percentCI 0.86 - 0.92) respectively for SE, SP, PPV and NPV. It can be concluded that professional flossing is a useful tool in the diagnosis of interproximal gingival inflammatory status in children, especially in conditions of gingival health.