Narrow dimensions of the entrance to furcations of multi-rooted teeth can complicate the periodontal management of furcation involvements. The objectives of this study were to measure the furcation entrance dimensions of first permanent molars from Hong Kong Chinese, and to compare these dimensions with those of the blade widths of periodontal instruments generally advocated for root surface instrumentation. A total of 363 first permanent molars, of which 185 were maxillary, were examined under 1.5 x magnification. Furcation entrance dimensions were measured using calibrated test gauges ranging from 0.3 mm to 1.5 mm. Furcation entrance dimensions equal to or less than 0.75 mm (the blade width of a new Gracey curet being 0.76 mm) in maxillary first molars were found in 79% of buccal entrances, 39% of mesial entrances, and 43% of distal entrances. In mandibular first molars, entrance dimensions equal to or less than 0.75 mm were encountered in 36% and 47% of buccal and lingual furcation entrances respectively. One-half of all furcation entrance dimensions of these first molars were less than the blade width of new Gracey curets. This high prevalence of narrow furcation entrances should be considered in the periodontal management of furcation involvement of first molars in Chinese.