Factors influencing gestational weight gain are incompletely understood, particularly among Hispanic women. We assessed medical, sociodemographic, behavioral, and psychosocial predictors of overall gestational weight gain, as well as gains below, within, or above the range recommended by the Institute of Medicine (IOM) within a prospective study of 770 Hispanic (predominantly Puerto Rican) prenatal care patients at a large tertiary care facility in Western Massachusetts. One third of women gained within the recommended range, 22% gained below, and 45% gained above the range. In multivariate analysis, women in the highest category of BMI (P(trend)<0.001) and parity (P(trend)<0.001) gained on average 9 lbs less than those in the lowest category. Increasing time in residence in the continental United States (P(trend)<0.01) as well as a number of prenatal care visits (P(trend)=0.03) were positively associated with weight gain. Overweight women (odds ratio (OR)=2.2, 95% confidence interval (CI) 1.3, 3.8) and those over age 30 years (OR=2.5, 95% CI 1.2, 5.0) were more likely to gain above the IOM range as compared to normal-weight women and those aged 20-24, respectively. Women with <10 years of residence in the United States were 50% less likely to gain above the IOM range as compared to third-generation women (95% CI 0.3, 0.9). Findings identify determinants of gestational weight gain which can form the basis of targeted interventions in this rapidly growing ethnic group.