In this study, we documented the prevalence of coronal axis malalignment in a series of 93 tibias (from 54 patients) lengthened with monolateral fixation. The average length obtained by distraction osteogenesis was 8.9 cm (range, 3.5-15.6) or 38% of the original bone length (range, 11-78%). Fifty (54%) of 93 tibias had documented valgus angulation of ≥ 10 ° or had fixator manipulation during the lengthening process for undesirable or progressive angulation; no cases of varus angulation were noted. Thirteen (14%) segments had later corrective osteotomy for unsatisfactory valgus malalignment. Statistical analysis revealed two factors to have a significant effect on the rate of malalignment. Those cases that had tibial osteotomy below the proximal one third of the original tibial length and those cases in which the fixator was placed >5 ° out of parallel had higher rates of angulation or manipulation (p < 0.001 and p = 0.002). Although the percentage of original bone lengthened was not statistically significant(p = 0.083), it did have an important effect on rates of axial malalignment. From this study we conclude that relatively high rates of malalignment in the tibia during distraction osteogenesis with monolateral external fixation are predominately the result of more distal osteotomies and nonparallel fixator placement. Attention to these details in general, and particularly where long lengthenings are planned, may significantly reduce this common complication.