The purpose of this study was to investigate the fine structural modifications of fibroblasts in the coronal region of inflamed human pulps from carious teeth. Six untreated human teeth with large carious lesions and two normal teeth as control were selected from male and female patients between the ages of 20 and 39. The teeth were divided into 4 groups by light microscopic findings: the normal control group, the chronic inflammatory cell-appeared group, the acute and chronic inflammatory cell-appeared group, and the total necrosis group. All tissues were fixed in 2.5% glutaraldehyde in 0.1M sodium cacodylate buffer at pH 7.4 and 1% osmic acid in same buffer. They were embedded in Epon 812. The ultrathin sections were stained conventionally and examined with a AEI Corynth 500 electron microscope. The results were as follows; 1. The fibroblasts of the normal pulps were almost in a quiescent state. 2. The active and the quiescent fibroblasts were found in the pulps of the chronic inflammatory cell-appeared group. Lymphocytes and plasma cells were also seen scattered among these fibroblasts. 3. In the pulps of the acute and chronic inflammatory cell-appeared group, active, degenerative and necrotic fibroblasts were found in the PMN appeared area. And all the fibroblasts in the fibrosis area were active. In the area of chronic inflammatory cellular infiltration, almost all the fibroblasts were active, but seldom were quiescent fibroblasts observed. Some fibroblasts in the pulps of two teeth had large vacuoles that contained banded collagen fibrils. The phagosomes had small beaded vesicles or large lysosome-like varicosity. In two of the teeth, microorganisms were present and two morphological shapes were identified, a rod and a coccus. 4. Vacuolar, vesicular, lamellar, fibrous and myelin structures were observed in the pulp of the total necrosis group, and cocci were also seen.