Fibrovascular tissue ingrowth into poly(vinyl alcohol) (PVA) sponges of different pore sizes was investigated by incorporating basic fibroblast growth factor (bFGF) into the sponges. The average pore size of PVA sponges used in this study was 30, 60, 110, 250, 350, and 700 mu m and gelatin microspheres were employed as release carrier of bFGF. The sponges were subcutaneously implanted into the back of mice after incorporating free bFGF or gelatin microspheres containing bFGF into the sponges. Fibrovascular tissue infiltrated with time into the sponge pores and the extent of fibrous tissue ingrowth showed a maximum at a pore size around 250 mu m 1 and 6 weeks after implantation. Significant promotion of the growth of fibrous tissue by bFGF was observed only at 3 weeks post-implantation (p < 0.05). New capillaries were formed in the tissue at any time, as long as bFGF was given to the sponges. Both empty gelatin microspheres and phosphate buffered solution neither promoted tissue ingrowth nor induced capillary formation in the sponges. It was concluded that bFGF was essential to induce the fibrovascular tissue ingrowth into the pores of PVA sponges. (C) 2000 Kluwer Academic Publishers.