Abstract: A diagnostic method for determining whether a suspect cell is cancerous using an optical gradient includes the steps of moving the suspect cell and the optical gradient relative to each other so as to cause displacement of the cell, measuring the displacement of the cell, comparing the measured displacement of at least one non-cancerous control cell. The step of comparing the measured displacement of the suspect cell and the at least one non-cancerous control cell determines whether the suspect cell is cancerous. The method can also be used to identify cancerous cells in a sample by identifying those cells having the largest measured displacements as a result of the relative movement between the cells and the optical gradient.