Oxidative damage is thought to play an important role in ischemia/reperfusion injury, including the outcome of transplantation of the liver and intestine. We have investigated oxidative DNA damage after combined transplantation of the liver and small intestine in 5 pigs. DNA damage was estimated from the urinary excretion of the repair product 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG). In the first 1–3 hr after reperfusion of the grafts, 8-oxodG excretion was increased 2.9-fold (1.7–4.1; 95% confidence intervals; P-1 kg-1 and 2½ hr, respectively. The injected dose was completely excreted into the urine within 4 hr. It is concluded that substantial oxidative damage to DNA results from reperfusion of transplanted small intestine and liver in pigs, as estimated from the readily excreted repair product 8-oxodG.