In a demographic sense, adolescents are a population group which is yet to take part in birth giving. Therefore, their sexual behavior and especially sexual activity at early ages which is not only unfavorable from the aspect of the individual, meaning risk for psycho-physical health, but from the aspect of society as well, as regards population fertility, deserves special attention. This paper shows the results of in-depth research carried out in Belgrade from September 2001 to October 2002 with an aim to establish which factors determine a young person, of sixteen years old or younger, to become sexually active. It was carried out on a sample of 111 adolescent girls between 14 and 20 years old which turned to the Republic Family Planning Center Youth Counseling Clinic of the Institute for Mother and Child Health Care of Serbia. The research showed that sexual experience, realized at an early age was an integral part of development and maturing for the largest number of surveyed girls. In the largest number of cases it was a positive experience, induced by love and experienced with a partner, mainly of the same age, with which they were in a longer, stable relationship. Nevertheless, it could be concluded from the results obtained by the research that the surveyed girls could have more easily and efficiently solved their problems and dilemmas regarding sexuality had they had the possibility to obtain a better insight into their personal feelings and feelings of others at the right time, as well as developed social experience and experience in mastering control of their impulses. With a certain number of surveyed girls that would have meant a delay in their sexual activities to a later age. This also refers to the prevention of other risky behavior such as use of alcohol and drugs, which also have an influence on changing sexual behavior, making it more risky. It is important to stress that the surveyed adolescent girls themselves recognized the necessity for support in this delicate field of life as well. The psycho-social profile of surveyed sexually active adolescent girls indicates that they do not belong under any circumstances to some marginal group. Therefore it can be concluded on the basis of the results obtained on this selected sample that an even greater necessity exists in the young population, and a possibility as well for better preparation and greater insight into questions from the sphere of sexuality. With a certain number of young girls this could lead to a certain delay in the commencement of sexual activities. Nonetheless, with a basic prerequisite of educational work on maintaining and promoting reproductive health of the young, which understands acquiring knowledge and skill, youth sexuality is above all, being accepted. Love takes up a significant place in the value system of our surveyed girls, as well as sex, which is mainly conceived as a constituent part of love, leading to a greater closeness with the partner and contributes to maturing and development of an individual. There is no reason to believe that it is otherwise in the population of the young people.