This study investigated differences in Vietnamese (native) and English (second) oral language proficiency across sex, age, socioeconomic status (SES), and grade levels for Vietnamese students as a result of their exposure to a 3-year (K-2) transitional bilingual education program. It also attempted to determine whether there was a difference in English language proficiency across Vietnamese language proficiency as well as whether there was a relationship between English language proficiency and Vietnamese language proficiency. One hundred twenty six subjects were selected based on the fact that they had participated in the said bilingual education program and that the scores they had obtained on the Language Assessment Scales (LAS) which measured their proficiency in English and Vietnamese were available for both kindergarten and second grade. These children's school records were examined in view of noting their LAS scores, sexes, birthdates, and SES. Statistical treatments used were analysis of covariance and Spearman's rho correlation. Tukey post-hoc tests were calculated where appropriate. The results indicated that: (1) for Vietnamese language proficiency, girls scored higher than boys, middle- and high-SES students scored higher than low-SES students, but no significant difference was found between the scores of older students and the scores of their younger counterparts; (2) for English language proficiency, females scored higher than males, older students scored higher than younger students, and scores increased linearly with SES increment; (3) there was a significant difference in English language proficiency across Vietnamese language proficiency; (4) there was a significant positive relationship between English language proficiency and Vietnamese language proficiency; and (5) the subjects enjoyed a marked gain in English but suffered a significant loss in Vietnamese.