Two patients developed an anterior tibial compartment syndrome after closed reamed intramedullary nailing of their fractured tibial shaft. Subsequently, a study was undertaken using a canine model to evaluate the risk of compartment syndrome after this operative procedure. A closed tibial fracture was created in 20 mongrel canines by applying a torsional load localized to the tibial shaft through a stress riser made with an intramedullary drill. Compartment pressures were measured in the canine anterolateral and deep posterior leg compartments. In 10 dogs, closed intramedullary nailing with reaming was performed while compartment pressures were measured. The remaining 10 dogs served as controls. Compartment pressures were monitored for 4 hours. In the experimental group, one dog attained and maintained a pressure greater than 50 mm Hg; a second dog maintained a pressure of 20 mm Hg during the postoperative monitoring period. In both of these cases, fasciotomy reduced the pressures to zero. The high tissue pressure readings were localized to the anterolateral muscle compartment. In the control group, no pressures higher than 8 mm Hg were recorded. Compartment syndrome is a potential complication of closed intramedullary nailing of the tibia, and the anterior muscle compartment appears to be the one most at risk. Therefore, perioperative tissue pressure monitoring is recommended.