We analyzed the Washington State births in 1980-1986 to Southeast Asian parents whose birth place was coded as being outside of the US. The mean birthweight increased from 3188 grams in 1980-81 to 3283 grams in 1986, an annual increment of about 18 grams (95% CI = 11, 25 grams). The prevalence of low birthweight (less than 2500 grams) decreased from 7.2 percent in 1980-81 to 5.4 percent in 1986, an annual reduction of 6.4 percent (95% CI = 1.3, 11.2 percent). In an analysis of information ascertained from the birth certificate, the change of paternal occupational status (from student to employed) was associated with 27 percent of the reduction in the prevalence of low birthweight, independent of maternal age, infant sex, and prior gravidity. A similar temporal change of birthweight during the same period of time was not observed among infants of US-born Asian mothers. Among the women who had two consecutive births from 1984 to 1986, the improvement of birthweight for the second birth compared to the first birth was much greater in infants born to the Southeast Asian mothers than that to White mothers. Our study suggests that living in the United States after immigration from their native countries has had a positive impact on the birthweight of infants born to Southeast Asian immigrants.