A discussion of the measures used to determine the failure rates of contraceptive methods precedes the presentation of a table that presents the theoretical and use failure rates for the following contraceptive methods: condom; coitus interruptus; diaphragm; IUD; oral contraceptive combined; oral contraceptive, progestogen only; vasectomy; tubal ligation; spermicidal foam; depo-provera; chance; lactation for 12 months; and the symptothermal method of natural family planning. A commonly used measure of contraceptive effectiveness is the Pearl Index. It measures the percentage of sexually active women who become pregnant while using a method for 1 year and can be expressed as either the minimum failure rate (theoretical failure rate) or as a maximum failure rate (use failure rate). Whether an individual or a couple operates at the minimum or maximum failure rate or somewhere in-between is dependent on: the basic contraceptive priorities of the method; the information provided by the practitioner for its use; and the extent of the user's compliance with the instructions. Theoretical failure rate is defined as the method's failure rate when the user receives correct instructions and follows these instructions conscientiously. The use failure rate is defined as the method's failure rate in actual use which includes: the user receiving incorrect instructions as to method use; the user forgetting to use the method sometimes or not using the method correctly. Theoretical failure rates listed in the table are: condom, 0.4-1.6; diaphragm, 2; IUD, 1-3; combined OC, 0.1; progestogen only OC, 1-2; vasectomy 0.1; tubal ligation 0.04; spermicidal foam, 3; depo-provera, 1; lactation for 12 months, 25; coitus interruptus, 9; and the symptothermal method -- variant A, 1.5; variant B, 3.39; and variant C, 11.2.