One's socio-economic status affects almost every aspect of his/hers life, including education. The main objective of this paper is to determine the relationship between familial socioeconomic status and students’ educational achievement. The paper is based on the OECD – PISA international study carried out in Croatia in 2006 on the sample of 5 209 15-year-old students involved in secondary education. Students’ results in scientific literacy, as well as variables and indices constructed by OECD, were used in analyses. The distributions of selected variables linked to socio-economic status definition, such as parental education, monthly household income, educational expenses and home educational resources are presented, as well as their regional differences. Multiple regression analysis proved the relationship between socioeconomic factors and students’ science test accomplishment. Regression model which includes three indices (Highest International Socioeconomic Index of Occupational Status, Index of home educational resources and Index of cultural possessions) and four variables (average monthly household income, average monthly educational expenses, household book number and highest educational status of parents) explain 23 per cent of the total variance of students’ achievement in scientific literacy. The second regression model with the region of residence additionally contributes to variance explanation by 1 per cent, indicating significant relation between living in specific Croatian region and science achievement. The main conclusion of this paper is that socio-economic status (along with the region of residence) represents a significant predictor of the educational achievement, and that it is relevant in analyses and interpretations of PISA results, even though Croatia could use an adaptation of PISA indices in order to trustworthy represent the data. It is advisable to continuously analyse the test results and, besides some general indicators of educational success, such as educational system, teacher qualifications and school infrastructure, also to consider the effect of students’ socioeconomic status.