Using a technique for simultaneous demonstration of G bands and SCE, we analyzed the SCE frequencies, cell cycle kinetics, distribution of SCE points and "hot spots" which correlate with fragile sites, proto-oncogenes locus in cultured peripheral blood lymphocytes in 20 untreated patients with nasopharyngeal carcinoma (NPC). The SCE frequencies were found to be higher in patients as compared with controls (P < 0.01). The SCE points of patients were significantly higher than those of controls in chromosome 1,2,3,4,5,7,8,12,13,14,17 (P < 0.05), indicating the chromosome instability of NPC patients. We identified 17 points as high frequency SCE hot spots, and found that 29.41% SCE hot spots fragile sites and proto-oncogenes were localized at the same bands. The distribution of SCE points and chromosome structure were discussed.