Confusion about the meanings of the terms infertility and sterility, which differ in medical and demographic usage and in the French and English languages, has led to inflated estimates of the number of couples who are sterile. In medical usage, sterility usually refers to inability to produce a live child, while infertility is measured by failure to conceive after 1 year without contraception. In demographic usage, sterility refers to nonsurgically-achieved inability to procreate, while (in English) fecundity refers to capacity to produce a live child and fertility and infertility refer to reproductive performance rather than capacity. Such statements recently encountered in the popular press and the scientific literature as "15% of couples are unable to have children" are proven false by survey and census data which show that the proportion of nonsingle women who are voluntarily or involuntarily childless is much lower. Difficulty in conceiving is more common than outright sterility. Data from preindustrial populations characterized by natural fertility as well as from contemporary populations which have discontinued contraception to generate pregnancies both indicate that significant numbers of women continue to become pregnant after more than 1 year without contraception, thereby casting doubt on the validity of the 1-year criterion for diagnosing infertility. A British prospective study of women 25 years old and over showed that the proportions of women not giving birth at 24, 36, and 42 months after discontinuing contraception were higher for oral contraceptives (OCs) than for other methods. After 24 months, 20.8% of nulliparous and 10.4% of multiparous women who had discontinued OCs had not given birth, compared to 15.5% of nulliparous and 8.1% of multiparous women who had used another method. Significant proportions continued to give birth through the end of the study. A different study, of women who had already given birth in a Boston hospital, indicated that the amount of time required for 90% of the sample to become pregnant after discontinuing contraception was 24 months after OC use, 14 months after IUD use, 10 months after diaphragm use, and 13 months after use of other methods. An American study of 597 couples treated for infertility and 548 "infertile" couples who were not treated suggested that the pregnancy rates of both groups were very similar. Only in 12.8% of the sample suffering from particularly severe clinical causes of infertility were pregnancies clearly obtained as a result of treatment. 66.9% of the other pregnancies were judged to have been obtained independently of treatment. Recent surveys and mathematical models of fecundability based on empirical data from noncontracepting populations indicate that, although the great majority of couples conceive within 1 year, the probability of conception after a year is far from null. Claims that sterility and difficulties in conceiving are on the increase because of sexually transmitted diseases, that increasing numbers are consulting physicians because of infertility, and that women are waiting longer to have children are difficult to judge with available data and merit further study.