OBJECTIVE To determine if there are differences in healing, complications, or number of procedures required to obtain union among open and closed tibia fractures treated with intramedullary (IM) nails inserted with and without reaming. METHODS Prospective, surgeon-randomized comparative study. METHODS Level One trauma center. METHODS Ninety-four consecutive patients with unstable closed and open (excluding Gustilo Grade IIIB and IIIC) fractures of the tibial shaft treated with IM nail insertion between November 1, 1994, and June 30, 1997. METHODS Interlocked IM nail insertion with and without medullary canal reaming. METHODS Time to union, type and incidence of complications, and number of secondary procedures performed to obtain union. RESULTS For open fractures, there were no significant differences in the time to union or number of additional procedures performed to obtain union in patients with reamed nail insertion compared with those without reamed insertion. A higher percentage of closed fractures were healed at four months after reamed nail insertion compared with unreamed insertion (p = 0.040), but there was not a difference at six and twelve months. More secondary procedures were needed to obtain union after unreamed nail insertion for the treatment of closed tibia fractures, but the difference was not statistically significant given the limited power of our study (p = 0.155). Broken screws were seen only in patients treated with smaller-diameter nails inserted without reaming, and the majority occurred in patients who were noncompliant with weight-bearing restrictions. There were no differences in rates of infection or compartment syndrome. CONCLUSIONS Our findings support the use of reamed insertion of IM nails for the treatment of closed tibia fractures, which led to earlier time to union without increased complications. In addition, canal reaming did not increase the risk of complications in open tibia fractures.