['We established a co-culture system to investigate endothelial cell-fibroblast interaction in scleroderma (systemic sclerosis, SSc). Such a system allows reciprocal interaction between these cells. The pattern of phenotypic modulation for normal and SSc fibroblasts in co-culture was compared.', 'A virally transformed human umbilical vein endothelial cell (HUVEC) derived cell line (1E-7) was cultured on nitrocellulose membrane inserts above dermal fibroblast monolayers. The effect of co-culture on fibroblast number, [3H]-thymidine ([3H]-TdR) incorporation, and collagen (type I) production were compared for 10 SSc and 5 control cell lines. Co-culture with the epithelial lines A549 and A431, and nontransformed HUVEC, was also investigated.', 'We observed a statistically significant increase in cell number and a reduction in collagen production for SSc, but not control, fibroblasts co-cultured with endothelial cells. This co-culture also promoted [3H]-TdR incorporation in both SSc and control fibroblasts. While epithelial cell lines did not influence fibroblast cell number, collagen production by SSc fibroblasts was diminished by A549.'] Is scleroderma fibroblast phenotype modulated by endothelial cell co-culture?