letter Xiao Hua Tao, Institute of Dermatology, Chinese Academy of Medical Sciences and Peking Union Medical College 18 September 2007 Sir,It was interesting reading “Chlamydial infection among patients attending STD and genitourinary clinics in Taiwan”published in BMC Public Health 2007, 7:120 . We have some queries which deserve to be further discussed:1. Perhaps the results section is confusing. In the table one, the proportions of genital skin rash/ulceration in chamydial positive group and in chamydial negative group are wrong. Moreover, patients with infection of T. vaginalis had a higher risk for chlamydial infection than patients without infection of T. vaginalis (P < 0.05), which has been missed by the authors. These data should need more classification. 2. The study found that concurrent gonococcal infection was significantly associated with chlamydal infection but no significant association was found with syphilis (from our opinion, not including T. vaginalis) or HIV infection. And the authors have given us possible explanations, such as lower rate of HIV infection and high sensitivity of the employed diagnostic Chlamydia trachomatis/Neisseria gonorrhoea PCR assay. In fact, much of cases with syphilis and HIV infection would be asymptomatic and it can be transmitted by blood besides genital tract. But, genital infections with Chlamydia trachomatis, Neisseria gonorrhoea or T. vaginalis would have the same symptoms and the same ways of transmission. Furthermore, the level of seeking care among these cases would be higher in STD and genitourinary clinics, which is a selection bias in the study that leads to a distortion of the results.Reference1. Chen KT, Chen SC, Chiang CC, Li LH, Tang LH. Chlamydial infection among patients attending STD and genitourinary clinics in Taiwan. BMC public health 2007, 7:120 Competing interests we have no competing interests with the authors.