Early and accurate detection of cancer often requires time-consuming techniques and expensive instrumentation. To address these limitations, we developed a colorimetric assay for the direct detection of diseased cells. The assay uses aptamer-conjugated gold nanoparticles to combine the selectivity and affinity of aptamers and the spectroscopic advantages of gold nanoparticles to allow for the sensitive detection of cancer cells. Samples with the target cells present exhibited a distinct color change while nontarget samples did not elicit any change in color. The assay also showed excellent sensitivity with both the naked eye and based on absorbance measurements. In addition, the assay was able to differentiate between different types of target and control cells based on the aptamer used in the assay indicating the wide applicability of the assay for diseased cell detection. On the basis of these qualities, aptamer-conjugated gold nanoparticles could become a powerful tool for point of care diagnostics.