Breast cancer (BC) survivors report adverse sexual effects such as disrupted sexual function (SF) and sexual distress. Despite its high prevalence, sexual dysfunction (SD) is not effectively screened for or treated. Population-based study with premenopausal BC patients, aiming to assess the prevalence and predictors of SD. Sexually active (SA) women (W), stage I-III, that have finished and recovered from chemotherapy, radiotherapy, and surgery were included. SF was assessed using the Female Sexual Function Index (FSFI), a 19-item instrument assessing 6 domains of SF: Desire, Arousal, Lubrication, Orgasm, Satisfaction, and Pain. The total FSFI cutoff score for a diagnosis of SD was ≤ 26.5 FSFI acceptability and assessment of sexual health during follow-up appointments was measured with Likert scales (punctuated from 1-5). FSFI questionnaire was completed by 181 W, 36 were excluded after confirming they weren’t SA and 145 W were included with a median age at diagnosis of 40 years (22-57). Eighty-three qualified (57,2%) for SD based on FSFI. A statistically significant difference was found in lubrication difficulties between W under LHRH agonists plus endocrine therapy (ET) and W who were only under ET (3,92 ± 1,39 vs.4,38 ± 1,30 p=0,049). FSFI scores were compared between surgery modality: Mastectomy (M), Mastectomy with immediate breast reconstruction (MIR) and Tumorectomy (T). Statistically significant differences were found between the 3 groups: M (23,50 ± 5,70), MIR (27,27 ± 4,93), T (22,86 ± 7,34) p=0,004. Post hoc analysis revealed statistically significant differences between M and MIR (p=0,009), T and MIR (p=0,01), however not between M and T (p=1,00). Out of the 145 W, 66,9% were rarely or never asked about sexual health. Overall, W provided positive feedback about FSFI: they felt comfortable answering the questionnaire (Likert scale punctuation media (LS) = 4,44); it was easy to complete (LS = 4,5), relevant to their experiences (LS = 4,39), and had the right length (LS = 4,42). Our study suggest that SD is a greatly underestimated problem in this population. We report substantial sexual morbidity, especially after M and T, while LHRH agonists plus ET were associated with lubrication difficulties. FSFI questionnaire was suitable and well accepted.