Background: The frequency of patients developing Breast Cancer (BC) among all other types of cancers exceeds 35% in Western Algeria. The main objective here was to determine which risk factors were associated with this high frequency of BC occurrence in young adult women (<40 years old) in Western Algeria. Methods: A BC case-control study matching with the woman age was conducted. The total number of participants was 484 women from Western Algeria. Inclusion criteria were BC and age <40 and women older than 40 or without documented BC were excluded. Results: Univariable analysis showed that i) there was an increased risk of BC; OR = 1.77 [1.06 to 2.93] in married women using oral contraception drugs; ii) in contrast there was a lesser risk; OR = 0.43 [0.28 to 0.65] of BC in multiparous compared to nulliparous women; iii) similarly there was a lesser risk; OR = 0.45 [0.26 to 0.76] of BC in married, divorced and widowed women compared to single women. Multivariable analysis showed a higher risk; OR = 2.34 [1.05 to 5.22] of BC in non-breastfeeding than lactating women. Conclusion: The use of oral contraception drugs, nulliparity, lack of breastfeeding and the single unmarried status are risk factors associated with increased BC in young adult women (<40) from Western Algeria.