Prospectively collected cycles of 207 women were used to find out the efficacy of the Cyclotest 2 plus algorithm in detecting the fertile time in a woman's cycle. The results of the device were compared with the beginning and the end of the fertile time identified by the symptothermal method (STM) of natural family planning (NFP). It was found that the algorithm led to dangerous reduction of the fertile time (FT) in only 2 out of 207 woman cycles (0.96%). However, at the end of fertile time (FE) the device requested more abstinence than was necessary in about 12% of the cycles. We feel that more research should be performed on detecting the end of the fertile time.