Objectives: This study assessed personal exposure of pregnant women to fine particles (PM_2.5) and benzo[a]pyrene (B[a]P) and the relationship between pollutant concentrations in ambient and indoor air. Materials and Methods: In a group of 78 pregnant women, simultaneous 48 h measurements of personal, indoor, and outdoor exposure to PM_2.5 and B[a]P were carried out in the second trimester of pregnancy. The results show that participants were exposed to varying concentrations of PM_2.5 and B[a]P, with higher exposure in the winter season. Overall, the mean personal PM_2.5 level was 30.4 μg/m³ and B[a]P 2.1 ng/m³. The winter/summer ratios for mean personal exposures were 1.4 (35.6 μg/m³ vs. 25.8 μg/m³) and 5.4 (4.9 ng/m³ vs. 0.9 ng/m³), respectively. As for indoor levels, the winter/summer ratios were 1.4 (33.2 μg/m³ vs. 24.4 μg/m³) for PM^2.5 and 5.4 (4.3 ng/m³ vs. 0.8 ng/m³) for B[a]P, and for outdoor concentrations, the respective values were 1.5 (40.3 μg/m³ vs. 26.4 μg/m³), and 6.8 (6.1 ng/m³ vs. 0.9 ng/m³). A stronger correlation was found between personal PM^2.5 exposure and the pollutant concentration indoors (r = 0.89; 95% CI: 0.83-0.93) than outdoors (r = 0.75; 95% CI: 0.64-0.83). The correlations between personal B[a]P exposure and its indoor or outdoor levels were similar (0.95-0.96) and significant. The markedly higher exposure to B[a]P in Kraków in winter than in summer can be explained by the massive use of coal for heating in the cold season. Conclusion: We conclude that although ambient PM_2.5 measurements provide an adequate indicator of outdoor air quality for use in epidemiologic studies, they may not be adequate for studies on relationship between non-ambient pollution and health effects. Since only about 20% of variability in personal B[a]P exposure could be explained by personal PM_2.5 level, the extrapolation of personal exposure to B[a]P from personal PM_2.5 data may be greatly underestimated.