Optical coherence tomography (OCT) was used to monitor the dynamics of tumour spheroid formation by the hanging drop method. In contrast to microscopy, the estimates obtained using OCT for the volume of the spheroid, were consistent with the measured changes in cell number as a function of time. The OCT images also revealed heterogeneous structures in the spheroids of similar to 200 mu m diameter. These corresponded to the necrotic regions identified by fluorescence of propidium iodide stained cells.