Jia et al. BMC Cancer 2010, 10:446 http://www.biomedcentral.com/1471-2407/10/446 medicinal herbs and NPC should be interpreted with caution, and further in-depth analyses and experiments are required. We have several limitations in this study. Similar to case-control approach, recall bias is inevitable. The data on intake frequency inevitably include a certain percen- tage of misclassification, particularly among older sub- jects when recalling the past. For example, when asked about past intake frequency, some people could not remember exactly whether it was monthly or weekly. Perhaps this inexact recall partly explains the observa- tion that the highest intake frequencies of salted fish and fermented paste (bean, fish or shrimp paste) were associated with lower odds ratios. In addition, we did not obtain the precise frequencies of dietary consump- tions, for example, how many times per day, and none evaluation for their contribution to NPC risk were pro- vided. Moreover, the study population may be aware of some of the known NPC risk factors such as salted fish. Thus, this factor may be another source of recall bias. Socioeconomic status (SES) could affect our subject recruitment in several ways. Recruitment of controls from people taking health examinations may also yield ascertainment bias towards higher SES in the controls. However, SES was carefully controlled not only in fre- quency matched design of cases and controls’ recruit- ment, but also in data analysis. With these steps, the confounding effect of SES was minimised to a great extent. However, the possibility that residual SES con- founding may exist cannot be excluded. Conclusions Taken together, this study has provided a clearer view of the dietary risk factors for NPC. Similar to previous stu- dies, consumption of salted fish and other preserved foods increased the risk of NPC, while consumption of fresh fruit reduced the risk. Further, we found that cases consumed less herbal tea and herbal slow-cooked soup than controls. Further study is needed to integrate mole- cular pathology with epidemiologic study for the identi- fication of the carcinogens. Acknowledgements We thank Li-Wei Wu, Ning-Hua Wu and Hong Deng for conducting the interviews and Ru-Hua Zhang, Xing-Juan Yu, Zhen-Dong Zhong, Zhe-Yu Hu, Jin-Fen Xu, Xin-Jian Li, Juan Wu, Xiao-Bin Lv and Yi Liang for entering and checking the data. This work was supported by the National Natural Science Foundation of China (30671798, 30471487), the National Science and Technology Support Program of China (2006BAI02A11) and the National Major Basic Research Program of China (863:2006AA02A404). Author details 1State Key Laboratory of Oncology in Southern China, Guangzhou, 510060, China. 2Department of Experimental Research, Sun Yat-Sen University Cancer Center, Guangzhou, China. 3School of Public Health, Sun Yat-Sen University, Guangzhou, 510080, China. 4Department of Dermatology, University of Utah School of Medicine, Salt Lake City, UT 84132, USA. 5Department of Page 6 of 7 Epidemiology, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD 21205, USA. Authors’ contributions WHJ is the guarantor of the study. She designed the study and was the main author of the manuscript. XYL performed data analysis and drafted the manuscript. BJF and HLR participated in data analysis and manuscript revising, JXB participated in data cleaning and analysis, WSL verified and cleaned the data, HDQ verified the data and revised the manuscript. QSF and LZC conducted sample collection and the interviews, SYY and YXZ participated in the design of the study. 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