Using 1970, 1980, and 1990 U.S. census data, the author examines the life-cycle patterns of immigrant women's labor force participation. He finds that the cross-sectional approach that has been used in all previous studies leads to a substantial over-estimate of the degree to which immigrant women's assimilation increases their labor force participation. The effect of assimilation found by using the cohort approach, however (which acknowledges the possibility that patterns of labor force participation partly reflect the year of immigration), is still sizable. The effect is concentrated within the first 10 years after arrival. There are substantial differences in participation and assimilation by country of birth. Immigrants from Japan, Korea, and China are found to have experienced the greatest degree of assimilation, with an effect that raises the probability of working by 20 percentage points during the first 10 years after arriving in the United States.