The contribution of brain edema to brain swelling in cases of traumatic brain injury remains a critical problem. The authors believe that cellular edema, the result of complex neurotoxic events, is the major contributor to brain swelling and that vasogenic edema, secondary to blood-brain barrier compromise, may be overemphasized. The objective of this study, therefore, was to quantify temporal water content changes and document the type of edema that forms during the acute and late stages of edema development following closed head injury (CHI). The measurement of brain water content was based on magnetic resonance imaging-determined values of tissue longitudinal relaxation time (T1-weighted imaging) and their subsequent conversion to percentage of water, whereas the differentiation of edema formation (cellular vs. vasogenic) was based on the measurement of the apparent diffusion coefficient (ADC) by diffusion-weighted imaging. A new impact-acceleration model was used to induce CHI. Thirty-six adult Sprague-Dawley rats were separated into two groups: Group I, control (six animals); and Group II, trauma (30 animals). Fast ADC measurements (localized, single-voxel) were obtained sequentially (every minute) up to 1 hour postinjury. The T1-weighted images, used for water content determination, and the diffusion-weighted images (ADC measurement with conventional diffusion-weighted imaging) were obtained at the end of the 1st hour postinjury and on Days 1, 3, 7, 14, 28, and 42 in animals from the trauma and control groups. In the animals subjected to trauma, the authors found a significant increase in ADC (10 +/- 5%) and brain water content (1.3 +/- 0.9%) during the first 60 minutes postinjury. This is consistent with an increase in the volume of extracellular fluid and vasogenic edema formation as a result of blood-brain barrier compromise. This transient increase, however, was followed by a continuing decrease in ADC that began 40 to 60 minutes postinjury and reached a minimum value on Days 7 to 14 (10 +/- 3% reduction). Because the water content of the brain continued to increase during the first 24 hours postinjury (1.9 +/- 0.9%), it is suggested that the decreased ADC indicated cellular edema formation, which started to develop soon after injury and became dominant between 1 and 2 weeks postinjury. The study provides supportive evidence that cellular edema is the major contributor to posttraumatic swelling in diffuse CHI and defines the onset and duration of the increase in cellular volume.