SummaryA case of lymphogranuloma venereum due to Chlamydia trachomatis serotype K is reported.A false clinical diagnosis of syphilis was made and penicillin therapy did not influence the evolution of the disease. Chlamydia trachomatis was isolated from an inguinal lymphnode on irradiated McCoy-cells.The micro-immunofluorescence test on the patients serum was strongly positive, 1/256 or more for serotypes K, L, and L3(IgG) and 1/32 for serotype K (IgM). A cryoglobulin could be demonstrated in the serum. The patient was treated with doxycycline 100 mg b.i.d. for 8 weeks, after which she was completely cured and her serology had become almost negative.