In humans, bilirubin levels are influenced by different factors. This study evaluates how several nongenetic causes and the genetic UGT1A1 polymorphisms contribute for bilirubin levels, in a cohort of 146 young Caucasian females. Hematological data, bilirubin, screening of TA duplication in the UGT1A1 gene, body mass index (BMI) and body fat were determined. A questionnaire about fasting time, smoking habits, oral contraceptive therapy, caloric intake and physical activity was done. Participants were divided according to the tertiles of bilirubin. Subjects from the second and third tertile had significant rises in hemoglobin (Hb), hematocrit, mean cell Hb and mean cell Hb concentration, and a significant increased frequency for the c.-41_-40dupTA allele in homozygosity, when compared to the first tertile. Red blood cell count was significantly increased in the third tertile. Results showed that the c.-41_-40dupTA allele (genetic), Hb, BMI and fasting time (nongenetic) were the main factors associated bilirubin levels.