The primary objective of the present study was to investigate the relationship between language proficiency and socio-cultural factors among the Iranian MA students registered and entered at Shiraz University in 2008. The second objective was to explore whether or not one or a combination of socio-cultural factors are predictors of language proficiency. Finally, the study aims at investigating the effects of gender, age and major on language proficiency. To fulfill the objective, a 100-item language proficiency test (30 items for grammar, 40 items for vocabulary and 30 items for reading comprehension) and a questionnaire including socio-cultural factors were distributed among 510 male and female Iranians taking part in the MA Placement Test at Shiraz University. The data gathered were analyzed descriptively utilizing central tendency measures (mean and standard deviation). Moreover, the collected data were analyzed inferentially using correlation, regression analysis, ANOVA and independent t-test. As such, the following results were obtained: a) although the descriptive statistics showed that females outperformed males in language proficiency, the independent sample t-test revealed that the difference is not significant; b) a significant difference was found between the participants' language proficiency in terms of their majors and their social classes; c) a moderate significant relationship was observed between the participants' language proficiency and their age; d) among various cultural factors, mother's literacy and using films/CDs predict language proficiency to a great extent. (Contains 7 tables and 1 figure.)