Although birthplace and length of residence have been found to be associated with Body Mass Index (BMI)/obesity in the USA, their effects may not be the same across groups defined by education, gender and race/ethnicity. Using cross-sectional population based data from the 2001 California Health Interview Survey, we investigated the associations of birthplace and US length of residence with BMI, and whether the influence of birthplace-US length of residence on BMI varied by education, gender and race/ethnicity. Our sample included 37,350 adults aged 25-64 years. Self-reported weight and height were used to calculate BMI. Birthplace and length of residence were combined into a single variable divided into five levels: US-born, foreign-born living in the United States for more than 15, 10-14, 5-9, and less than 5 years. Controlling for age, gender, marital status, race/ethnicity, education, income, fruit and vegetable consumption, current smoking and alcohol use, we found that: (1) foreign-born adults had lower BMI than US-born adults; (2) among foreign-born adults, longer residence in the United States was associated with higher BMI; and (3) the effect of birthplace-length of US residence on BMI differed by education level, gender and race/ethnicity. Specifically, longer residence in the United States was associated with the greatest percent increases in BMI among the lowest educated groups than higher educated groups, among women (vs. men) and among Hispanics (vs. other racial/ethnic groups). These findings suggest that a protective effect of foreign birthplace on BMI appears to attenuate with length of residence in the United States, and also reveal that BMI/obesity trajectories associated with length of US residence vary by education, gender and race/ethnicity. Immigrant status, independently and in combination with education, gender and race/ethnicity should be considered in future obesity prevention and reduction efforts.