Microglia affect and are sensitive to events in the central nervous system, changing in morphology and function as they respond to and resolve disruptions. Monitoring microglia is, therefore, an important goal of neuroscience. We investigated morphological changes in cultured mouse microglia, using the box counting fractal dimension (DB), lacunarity, and other measures. The DB and lacunarity corresponded well to visually applied classification systems of these cells. Complementing such systems, which depend on grossly visible differences between cells, the DB also differentiated between visually indistinguishable microglia in different functional states (ie, deramifying versus reramifying). The results suggest that fractal analysis may help determine if a microglial cell is' resting? rousing itself for action, acting subtly, or returning to a resting state. Because of the awesome potential microglia have to affect events in the central nervous system, the implications of this for the study of human health and disease are profound.