Introduction: Lady-Comp, Baby-Comp and Pearly cycle computers are medical devices that use sophisticated statistical gathering methods, as well as a comprehensive database, to precisely determine fertile and infertile phases of a menstrual cycle on the basis of everyday basal body temperature measurements. They have been produced and distributed worldwide by Valley Electronics GmbH (Eschenlohe, Bavaria, Germany) for over 25 years. Objectives: The aim of the study was to calculate the Pearl Index of cycle computers in order to determine their contraceptive effectiveness. Material and methods: 510 Polish women, randomly chosen from the database of the distributor, who had been using the device for over one year or during 13 menstrual cycles, received the questionnaire. The Pearl Index was calculated as a quotient of the number of unplanned pregnancies and the total number of cycles during which cycle computers were used and the obtained value was then multiplied by 1300. Statistical methods were applied to analyze data from the questionnaires and to calculate the Pearl Index. Unplanned pregnancy odds ratio for women using additionally condoms during the fertile phase of the cycle was also calculated. Results: 139 properly filled questionnaires were the source of data about 3332 cycles. After the initial analysis, 290 cycles were declined because the respondents had not complied with the computer indications and 1021 cycles were declined because the respondents had been using other contraceptive methods at the same time – no unplanned pregnancy was noted in that group. In the investigated group of 2040 cycles of correct cycle computers use, one unplanned pregnancy was observed. Calculated Pearl Index for this group amounted to 0.64; it means, that less than 7 out of 1000 users of cycle computer as a contraceptive method may become pregnant within one year. The odds of pregnancy in women using a cycle computer and condoms on fertile days amounted to 1.035%; it means that 1 out of 100 users of the combined methods may become pregnant within one year. Conclusions: The Pearl Index value of cycle computers is comparable with the Pearl Index of hormonal contraceptives. Cycle computers offer an effective and drug-free method of contraception to all women who wish to limit interventions in their bodily functions and do not want or cannot use other contraceptive methods.