To evaluate the indices used to compare the results of limb lengthening with distraction osteogenesis, we analyzed the relationship between the lengthening and external fixation period in a series of 28 patients (30 limbs). The average lengthening was 4.6 cm (range, 2.0-8.9). The average age at operation was 16.6 years (range, 4-47). Limbs treated included 15 femurs and 15 tibias of 19 male and 9 female patients. The external fixation period and the maturation period had a direct linear relationship with length gain. There was a negative hyperbolic relationship between the external fixation index, maturation index, and length gain. Segments that were lengthened 0-3.0 cm had a maturation index of 38.3 days/cm and external fixation index of 57.6 days/cm, both of which were significantly higher than for those lengthened more than 3.0 cm. When comparing the maturation and external fixation indices between different patients or studies, the length of the distraction must be taken into consideration.