This article is a commentary on a study published in the journal *Environmental Health Perspectives* 2000;108:647-654. In the late 1980s, observations made in Teplice (Northern Bohemia, Czech Republic), a highly polluted city, indicated a possible decrease in the birth rate during periods of high concentrations of sulfur dioxide in the ambient air. This potential association between pollution and a marker of fertility (number of children) prompted the authors of the present study to characterize the association between air pollution and fertility. A couple’s fertility is the probability of pregnancy during a menstrual cycle without contraception; it depends on several stages of reproductive function and, in particular, on the proper progression of gametogenesis in women and men, the transport of sperm to the egg, fertilization, implantation, and embryonic survival during the first weeks following conception (early miscarriages not clinically detected). Impaired fertility may be a sign of a disruption in one of these stages. Certain environmental factors have been shown to influence fertility, but the impact of air pollution had never been studied. To our knowledge, this study is one of the first to describe the association between exposure to an air pollutant and fertility; its main limitation lies in the choice of the exposure window, which is not suited to the event under study. The authors examine the probability of pregnancy during the first month without contraception in relation to atmospheric SO2 levels two months prior to conception. While the choice of this exposure window is relevant for couples who conceived within the month following the cessation of contraception, it is not relevant for those who took longer to achieve pregnancy. Due to a significant approximation in the definition of the exposure window, this study provides little insight into a potential effect of air pollution on fertility. The existence of studies indicating a possible effect of air pollution on sperm characteristics suggests that this question remains open.