Objective: The aim of this investigation was to compare the cell proliferative effects of a range of doses of low level laser therapy (LLLT) at wavelengths of 780, 830 and 904 nm on human breast and immortalised human mammary epithelial cell lines in vitro. Background Data: LLLT is used in the clinical treatment of post-mastectomy lymphoedema, despite safety information being limited and circumstantial. This research was the first step in systematically developing guidelines for the safe clinical use of LLLT in the management of post-mastectomy lymphoedema. Materials and Methods: Human breast adenocarcinoma (MCF-7), human breast ductal carcinoma (MDA-MB-435S) and immortalised human mammary epithelial (SVCT and Bre80hTERT) cell lines were irradiated with a single exposure of laser at 0.5, 1, 2, 3, 4, 10 and 12 J/cm2 (?=780 nm) and 0.5, 1, 2, 3, 4, 10 and 15 J/cm2 (?=830 and 904 nm). MCF-7 cells were further irradiated with two and three exposures of all three laser wavelengths. XTT colorimetric assays were utilized to assess cell proliferation 24 hours after irradiation. Results: SVCT cell proliferation significantly increased after exposure to a range of doses at 780 and 904 nm irradiation. MDA-MB-435S and Bre80hTERT cell lines showed negligible effects with one exposure from all three wavelengths and no dose response relationships were noted. MCF-7 cells irradiated with 780 nm laser demonstrated an increasing dose response relationship after one exposure and a decreasing dose response relationship after three exposures. The MCF-7 cells irradiated with 904 nm laser demonstrated a decreasing dose response relationship after two and three exposures. Conclusion: Despite certain doses of laser increasing MCF-7 cell proliferation, multiple exposures had no effect or a decreasing effect on dose response relationships. Before a definitive conclusion can be made regarding the safety of LLLT for post-mastectomy lymphoedema, further in vivo research must be conducted.