INTRODUCTION: Generaly, natural killer cells (NK cells) are among the most important cells of our immune defense system. They are present in the blood, decidua but also in secretory endometrium. OBJECTIVE: We investigate an association between high density of NK cells CD56+ and NK cells CD16+ into blood and secretory endometrium in patients with up to now unexplained recurrent miscarriage. At the same time, we focused on diagnosis of antiphospholipid syndrome, congenital trombophilic factors, stress factor, and lenght of hormonal contraception before conception. DESIGN: Original work-prospective study. SETTING: Genetics-Pilsen. METHODS: We investigated 59 patients aged 25-41 (average 34) years with their history of 3-9 abortions. We monitored their blood cells by flow cytometry and endometrium obtained by hysteroscopy performed on the 22nd-24th day of the menstrual cycle. We concentrated on the cellular immunity focused on the identification of lymphocytes CD56+ and CD16+. ELISA method was used for identification of antiphospholipid antibodies. RESULTS: Our study goes on, we are publishing our preliminary results. We found a high density of endometrial NK cells CD56+ in 41/57, NK cells CD16+ in 40/57, both NK cells CD56+ and CD16+ in 36/57 patients. In 13 women, we identified only sporadically these cells in the secretory endometrium. NK cells CD56+ and NK cells CD16+ in the blood were all normal number in all our patients. Three of them have a homozygot form of Leiden mutation, and 18/57 primary antiphospholipid syndrome, 12 women have university education, four of them, medical doctors, have regular night services in the hospital. CONCLUSION: Overproduction of endometrial NK cells is associated with increased local activity of embryocytotoxic cytokines that may negatively affect pregnancy. Treatment of patients with pathological immunological findings must be solved individually, as we will show soon in part two of our study.