Declining trends in male proportion at birth observed in several Western countries might reflect widespread exposure to pollutants capable of interfering with human reproduction. In this study, the authors describe male live birth proportion trends in 23 European countries from 1950 to 1996 (total of 305 million live births). Overall, there was a significant linearly decreasing trend of 10 fewer males per 100,000 births each year, resulting in a loss of 73,462 boys during a 47-yr period. The proportion of male births during the first 3 yr of the study period was higher than in the last 3 yr in 18 countries (i.e., 78%). Decreasing trends, which varied in slope and shape, were observed in 11 countries; no significant trend was found in 8 countries, and male birth proportion increased in 4 countries. The results of this study confirmed that the proportion of male births is declining in Europe, and differences exist by region and country. Social and cultural aggregations of countries with decreasing trends suggest that sociodemographic characteristics might be more likely to explain trends than environmental exposures to chemicals. Investigators should evaluate this hypothesis to assess the usefulness of male birth proportion as a sentinel event.