We present a study of the three-dimensional structure of cancer cells using dual-wavelength phase-imaging digital holographic microscopy. Phase imaging of objects with optical height variation greater than the wavelength of light is ambiguous and causes phase wrapping. By comparing two phase images recorded at different wavelengths, the images can be accurately unwrapped. The unwrapping method is computationally fast and straightforward, and it can process complex topologies. Additionally, the limitations on the total optical height are significantly relaxed. This new methodology is widely applicable to other phase-imaging techniques as well as in applications beyond optical microscopy. © 2011 Optical Society of America.