We have evaluated the numbers and immunohistochemical positivity for tumor necrosis factor (TNF) alpha of the mast cells in (a) 40 skin samples collected at autopsy from subjects who had survived for a few seconds to 1 h, (b) 10 samples of post-mortem skin lesions and (c) 10 surgical biopsies of healthy skin. Sections were treated with fluoresceinated avidin, to tag mast cell granules, followed by indirect immunohistochemistry for TNF-alpha with polyclonal primary and rhodaminated secondary antibodies. We could confirm a progressive increase in mast cell numbers, which became significant 1 h after trauma. More important, the fraction of mast cells positive for TNF-alpha increased progressively in the same time period and became significantly higher than controls in specimens collected more than 15 min after trauma. Samples from post-mortem lesions had significantly fewer mast cells and fewer TNF-alpha-positive cells than any other group of samples. The results suggest that mast cells are quickly recruited to an injured site in response to trauma and upregulate their TNF-alpha content, which can play an early role in directing tissue response to injury. The forensic pathologist can take advantage from this behavior of mast cells for the evaluation of the timing of early vital lesions.