The CD34 positive (CD34+) spindle cells constitute a special population of spindle cells which shows a unique distribution in the skin. So far, however, the functional role of CD34+ spindle cells and the regulation of CD34 expression on dermal spindle cells are totally unknown. We examined immunohistologically the pattern of the expression of CD34 and proline-4-hydroxylase, a marker for the fibroblasts that participate in active collagen synthesis, on dermal spindle cells at various stages of scar and keloidal tissues. Dermal spindle cells in the lesions of hypertrophic scar and those at inflammatory expanding borders of keloids totally lost CD34 expression, but they strongly expressed proline-4-hydroxylase. On the other hand, they expressed CD34, together with decreased immunoreactivity to anti-proline-4-hydroxylase antibody, in non-inflammatory scars or in a non-inflammatory central portion of keloid. In two cases of scars, in which inflammation began to subside, double immunofluorescence demonstrated that both CD 34 and proline-4-hydroxylase were expressed on the same spindle cells. CD34 expression, once disappeared from the lesions of hypertrophic scar or keloid, seems to return on CD34-proline-4-hydroxylase+ cells, when the initial inflammatory changes begin to regress. There is a reverse correlation between CD34 expression on spindle cells and the synthesis of type I collagen in the skin.