The alkali-silica reaction (ASR) produces an expansive gel that may cause cracking and displacement in concrete structures. Steel microfibers ranging from 1% to 7% by volume of cement mortar were incorporated to reduce the expansion and cracking. All specimens contained 5% of opal by weight of fine aggregates. The samples were cast and tested according to ASTM C-1260. A considerable reduction in expansion was observed for all steel microfiber-reinforced mortar specimens compared to the control specimens without fibers. The higher the fiber volume fraction, the lower the expansion. At constant fiber volume fraction, the expansion was further reduced when the curing time was extended from 1 day to 7 days due to an increase in the fiber-matrix interfacial bond strength. (C) 2001 Elsevier Science Ltd. All rights reserved.