How much of the increase in sex ratio (male to female) at birth since the early 1980s in China is attributed to increased prenatal sex selection? This question is addressed by exploiting the differential introduction of diagnostic ultrasound in the country during the 1980s, which significantly reduced the cost of prenatal sex selection. We find that the improved local access to ultrasound technology has resulted in a substantial increase in sex ratio at birth. Our estimates indicate that roughly 40 to 50 percent of the increase in sex imbalance at birth can be explained by local access to ultrasound examinations. (Contains 9 tables, 8 figures, and 29 footnotes.)