Objective: To determine the knowledege of women about lactational amenorrhoea and contraceptive properties of breastfeeding. Design: A prospective, randomised descriptive study. Setting: Kocaeli University School of Medicine,: Department of Obstetrics and Gynecology. Subjects or participants: Nine hundred and twenty two: Women in their reproductive ages. Intervention: A questionnaire was filled by doctors :or nurses during face to face interview. Main outcome measures: There was significantly less: knowledge for the importance of frequency and duration of suckling (p< 0.0001). The education increases the knowledge of lactational amenorrhoea as a interruptus contraceptive method. Results: More than fifty three per cent of women were using one of the modern contraceptive methods, 23.86% were using natural methods and 22.78% not using any family planning method. Intrauterine devices (30.15%), coitus interuptus (21.69%) and condom (16.48%) were the most common contraceptive methods. Nearly fifty two per cent of women were not aware of the contraceptive property of breastfeeding, 25.68% of women knew lactation had a protective effect from-pregnancy, 48.16%, did not know the importance of frequency and duration of suckling on fertility reducing effect of lactation. Conclusion: The level of knowledge on lactational amenorrhoea and frequency of suckling was significantly low in our study, especially in the illiterate group. Since efficacy of natural family planning depends on the compliance of women, education of women about lactation is very important, Family planning programmes should be focussed on breastfeeding and type of breastfeeding practices used, especially where there are no contraceptive alternatives.