This study included 25 patients with lower limb tumors who had reconstruction by vascularized fibula performed at least 24 months before the end of the study. The average age at operation was 23.5 years (range 11-50).The resulting defect after tumor resection averaged 16 cm (range 9-20). The fibula was inserted as a single strut in 21 patients and as a double-barrel construct in 4 patients. Fixation of the graft was augmented by interlocking nails in 11 cases, bridge plates in nine cases and external fixators in five cases.On serial radiographs,the relative width of the fibular graft to the recipient bone was estimated by a modification of Deboer and Wood's formula. Significant graft hypertrophy (defined as > 30% of the original fibular width) was observed in 90% of the patients at an average follow up of 27 months (range 30-200%). The time to graft union (start of partial weight bearing) positively correlated with the time to significant graft hypertrophy (r=0.9, p<0.01). The time to significant graft hypertrophy showed no significant differences between patients as regards age, graft length, reconstruction site, fixation method and chemotherapy administration. No significant increase in the amount of graft hypertrophy was observed after plate removal or dynamization of locked nails. The final amount of graft hypertrophy was affected by the age of the patient (p<0.01) and the length of follow up (p<0.05). Graft hypertrophy progressed at an average rate of 3.3% per month (range 2.3 to 4.9%) until the end of the 30th month, thereafter little or no increase in graft hypertrophy was observed. The rate of graft hypertrophy showed two significant peaks at 6-12 months (p<0.001) and at 18-24 months (p<0.05). Patients younger than 20 years of age showed faster hypertrophy with a peak at the 12th month, while older patients showed slower but steadier rate. Hypertrophy progressed faster in patients who received chemotherapy until the 12th month then declined sharply compared to patients who did not receive chemotherapy. Otherwise, hypertrophy progressed at similar rate irrespective of the patients'sex, graft length, reconstruction site, and fixation method. In summary, hypertrophy of the vascularized fibular graft is a time-related phenomenon, which is basically dependent on mechanical loading of the graft. It is generally affected by age of the patient as younger patients have tendency for earlier and faster graft hypertrophy that is probably related to their higher remodeling power and increased level of activities.