In this study we compared the effects of compression on the maturity and pattern of osseous healing in canine tibial osteotomies that were fixed with an external fixation system at a single time-period, ninety days after osteotomy. The in vitro model demonstrated that compression increased rigidity of fixation. Relative to the rigidity of the intact tibia, this increase was small. Estimation of osteotomy-site bone blood flow at ninety days by 85Sr clearance and torsional testing to failure of the healed osteotomies were used to compare the maturity of bone union in each group. The pattern of healing of the osteotomy was evaluated by quantitative histological determination of new-bone formation and porosity. All of the osteotomies healed by the ninetieth day. We found no significant difference at that time in osteotomy-site bone blood flow or strength of healing of the osteotomy, although the compression-treated osteotomies healed with higher torsional stiffness. No differences were demonstrated between the two groups in the pattern of healing of the osteotomy. In both groups there was a significantly greater amount of periosteal new-bone formation in the relatively less rigidly fixed antero-posterior plane when compared with the mediolateral plane. Primary bone-healing of both the contact type and the gap type was seen in both groups. There were no significant complications and the pin-loosening rate was similar in both groups.