We present a viable circulating tumor cell (CTC) isolation method based on hypo-osmotic swelling, which is applicable to various size-based CTC isolation devices. The previous CTC filtration devices suffered from the size overlap between CTCs and large leukocytes. Affinity-based size enhancement has been employed to separate CTCs and leukocytes with similar sizes, but the size enhancement was confined to the CTCs expressing specific surface proteins and the cell loss or viability reduction was inevitable when detaching the antibody-conjugated beads from the captured CTCs. In contrast, hypo-osmotic swelling is applicable regardless of the cancer cell types. The size increments of both epithelial-and mesenchymal-like cancer cells were larger than that of leukocytes, with less than 10% of cell death at the osmolality of 190 mOsm kg(-1). Consequently, cancer cell isolation was 1.2-fold enhanced with negligible reduction in specificity or cell viability, when using one of the conventional CTC filters. We further explored the improvements in CTC isolation using patients' blood samples and confirmed that the CTC detection rate was enhanced when the samples were processed under hypotonic conditions. Our label-free cell size increment technique can be widely applied to the various CTC filters, for enhancing the isolation of heterogeneous CTCs.