A 4-month-old girl presented with sexual development, including breast enlargement, menstruation, uterine length of 69 mm at ultrasonography, and dramatically high estrogen bioactivity, but no growth acceleration, pubic hair, pelvis masses or adrenal tumors. Gas chromatography with an electron capture detector and mass spectrometry detected pesticides (p,p'-DDD, p,p'-DDT, lindane and endosulfan sulfate) in plasma from the infant, the mother, and the 38-year-old father, who reported a dramatic decrease in libido, and in soil samples from their farm. The precocious sexual development was probably caused by the estrogen activity of the environmental contamination by tons of pesticides stored in the family farm.