PIP: A 2-year prospective study of the sympto-thermal method of rhythm was conducted. The primary objective was the statistical evaluation of the biological effectiveness of sympto-thermal rhythm. The study was international in scope, comprising over 1000 couples living in 5 countries of the world (Canada, Colombia, France, Mauritius, and the United States). The image many people have of rhythm users is that of older couples with 5 or 6 children desperately trying to avoid another child. Analysis of the study participants shows otherwises. The age of the wives on January 1, 1971 varied from 20-44, and the size of the families was from 1-9 with the curve skewed heavily to the left. Only 205 of 1014 couples registered in the study failed to complete the full 24 months. During this time there were 236 pregnancies, 103 of which were planned and 133 unplanned. Most countries had a use effectiveness of between 6 and 7.5 conceptions/100 woman years. Colombia was the notable exception with slightly over 22 conceptions/100 woman years. For those using sympto-thermal only, 9 of 110 unplanned pregnancies were judged to be due to method failure and 101 to user failure. The user effectiveness is 7.18 pregnancies/100 woman years of exposure, and the biological effectiveness rate is 0.64 pregnancies/100 woman years of exposure.