There is an increasing interest for the use of nanostructures as potential tools in areas that include biology and medicine, for applications spanning from cell separation to treatments of diseases. Magnetic nanoparticles (MNPs) have been the most widely studied and utilized nanostructures in biomedical applications. Despite their popularity, the regular shape of MNPs limits their potential for certain applications. Studies have shown that magnetic nanowires (MNWs), due to their high-­-aspect ratio and specific magnetic properties, might provide improved performance for some biomedical applications. As a consequence, MNWs have received increasing attention from researchers in the last years. However, as with any other nanostructure intended for biomedical applications, rigorous studies must be carried out to determine their potential toxicity and adverse effects before they can be successfully incorporated in clinical applications. This work attempts to elucidate the cytotoxic effects of nickel NWs (Ni NWs) in human fibroblasts by measuring cell viability under different parameters. Ni NWs of three different lengths (0.86 ± 0.02 μm, 1.1 ± 0.1 μm and 6.1 ± 0.6 μm) were fabricated by electrodeposition using porous aluminum oxide (PAO) membranes as templates. Energy dispersive X-­-Ray analysis (EDAX) and X-­-Ray diffraction (XRD) were used for the chemical characterization of the Ni NWs. Their physical characterization was done using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) imaging. MTT assays were performed to assess cell viability of human fibroblasts in the presence of Ni NWs. NW length, NW/cell ratio and exposure time were changed throughout the experiments to elucidate their effects on cell viability. The results showed that NWs length has a strong effect on internalization and cytotoxicity. Smaller NWs showed higher toxicity levels at earlier times while longer NWs had stronger effects on cell viability at later times. NW/cell ratio did not seem to have a very strong effect at low concentrations. However, at high concentration (1000 NW/cell) significant loss of cell viability was observed, with the effects becoming stronger at later times. Other factors such as cell surface area, presence of oxide layer on NWs, and the cytotoxicity of Ni salts, were also studied and found to affect cell viability. For our knowledge, this is the first systematic study done in human fibroblasts wi-­-38 using ferromagnetic NWs; where the toxic effects of equivalent amounts of Ni in its salt and in its NW form are compared. It is also the first study to provide insights of the interaction between wi-­-38 cells and Ni NWs. The results of this study complement and enrich previous cytotoxicity studies of Ni NWs. This work aims at providing a more comprehensive understanding of the interaction between NWs and biological systems. Despite the advancements, further studies will be required to fully understand the factors affecting NW cytotoxicity. Only when we understand the underlying mechanisms, will we be able to design suitable nanostructures for biomedical applications.