Background: The important role that the immune system plays in malignant diseases is well known. The action of interleukin-7 (IL-7) as a cytokine has been observed in many cellular processes, both in normal cells of the immune system and in some cancer cells. The aim of this study has been to explore whether there is any elevation of interleukin-7 serum levels in early invasive breast cancer (EIBC) patients in comparison with healthy controls. In addition, the correlation between the IL-7 serum level and the histopathological characteristics of the tumor has been evaluated. Methods: This cross-sectional, observational, and analytical study included 213 consecutive patients with EIBC (113 from Croatia and 100 from Kosovo) and 62 healthy participants as the control group (30 from Croatia and 32 from Kosovo). Blood samples have been taken from patients confirmed with breast cancer (BC) by biopsy, prior to surgical intervention and other oncological treatments, as well as from healthy participants. A serum IL-7 level has been measured, using the "Sandwich" ELISA Immunoenzyme test. In addition, after the surgical intervention, histopathological specimen examinations and immunohistochemistry have been performed and analyzed. The differences in the distribution of the numerical variables have been analyzed with the Mann-Whitney U test and Kruskal-Wallis ANOVA test. Correlations have been tested with Pearson coefficients. A P-value < 0.05 has been accepted as statistically significant. Results: The serum level of IL-7 in EIBC patients was significantly higher than in control cases (P 0.001). Patients with invasive lobular carcinoma (ILC) seem to have a lower IL-7 serum level compared to other histological subtypes, and the difference has been significant (P = 0.043). There has been no correlation between IL-7 serum level and histopathological characteristics of the tumor, with neither age nor menopausal status of the patients. Conclusions: Noting the significant increase in the IL-7 serum level in the EIBC patients as compared to the healthy control group, the use of IL-7 as a potential diagnostic indicator for BC, as well as in the follow-up of the patients after treatment, can be assumed. The lack of correlation with tumor size, lymph node metastasis, and all other histopathological characteristics of the tumor questions its use as a prognostic indicator.