Paraoxonase 1 (PON1) is an enzyme with multiple activities, including detoxification of organophosphates. It is believed to be important in preventing neurotoxic damage and has also been implicated in atherosclerosis. The PON1 gene contains five common polymorphisms, three in the promoter (-909G > C, -162A > G, -108C > T) and two in the coding region (M55L, Q192R) with varying but incomplete linkage disequilibrium. Our previous study showed that functional polymorphisms in PON1 were strongly associated with enzymatic activity in both pregnant women [26-30 weeks of gestation] and neonates. However, there was substantial overlapping of enzyme activities between genotypes. In this study, we investigated whether haplotype (genotype + phase) information would strengthen the genotype-phenotype relationship for PON1. The study consisted of a multiethnic population of 402 mothers and 229 neonates. Haplotypes were imputed by two widely used programs, PHASE and tagSNPs, which yielded very similar results. There were seven haplotypes with a frequency of 5% or higher in at least one ethnic group of the study population. Haplotype composition varied substantially with respect to ethnicity. Haplotypes in Caucasians and African-Americans showed the largest difference, and Caribbean Hispanics seemed to be a mixture of Caucasian and African ancestry. Collectively, the genetic (genotype or haplotype) contribution to PON1 enzymatic activity (measured as phenylacetate hydrolysis) was greater in neonates compared with mothers. Specifically, 16.6% of PON1 variability was explained by genotypes in mothers compared with 30.9% in neonates. Haplotype information offered a slightly increased power inpredicting PON1 activity; they explained 35.5% and 19.3% of PON1 variability in neonates and mothers, respectively.