Cell anchorage is required for cell proliferation of untransformed cells, whereas anchorage-independent growth can be induced by oncogenes and is a hallmark of transformation. Whereas anchorage-dependent control of the progression of the G ₁ phase of the cell cycle has been extensively studied, it is less clear whether and how anchorage may control other cell cycle phases and whether oncogenes may affect such controls. Here, we found that lack of cell anchorage did not influence progression through the cell cycle S phase, G ₂ phase, or most of mitosis of primary human fibroblasts. However, unanchored fibroblasts could not complete cytokinesis. The cleavage furrow and central spindle were still formed in the absence of anchorage, but cells were unable to complete ingression, causing binucleation. Importantly, V12 H-Ras-transformed fibroblasts and two cancer cell lines progressed through the entire cell cycle without anchorage, including through cytokinesis. This indicates that oncogenic signaling may contribute to anchorage-independent growth and tumorigenesis by promoting the final cleavage furrow ingression during cytokinesis.