Objective To evaluate the changing pattern of pelvic incidence (PI) during peak skeleton growth in adolescence and to study whether PI is associated with the anatomical morphology changes of pelvis and sacrum.Methods This retrospective longitudinal study recruited adolescent idiopathic scoliosis (AIS) patients,age between 9 and 18 years with full spine images and have at least 3 times' follow-up.The radiological anatomical parameters were measured in each follow-up including PI,pelvic height (PH),pelvic width (PW),sacral width (SW),femoral head-sacrum (FH-S),sacrum-coccyx (S-C) length,sacrum-coccyx (S-C) distance and sacral curvature ratio.△Parameter and growth velocity of each parameters were calculated.According to the skeletal maturity,all subjects were divided into 3 groups:Low Risser Group (first visit at Risser 0 and follow up through Risser 0 to 1),Moderate Risser Group (first visit at Risser 1 and follow up through Risser 1 to 3) and High Risser group (first visit at Risser 3 and follow up through Risser 3 to 5).Each group has 106 patients.We increased the male ratio to detect the gender difference.A total of 318 AIS patients were included in our study and there are 117 male patients.Descriptive analysis was conducted on the sample age,gender and measured parameter values with mean and standard deviation (SD).The intra-and inter-observer reliability analysis,and the intraclass correlation coefficient (ICC) were calculated.The correlation between △PI and △parameter was evaluated by calculating Pearson correlation coefficient R value.Results The mean age of all subjects was 12.86 years and the mean Cobb angle was 26.67° at first visit.The peak growth velocity of PI was in Risser I (female 1.5°/year and male 1.6°/year).The PI was also increasing rapidly in Risser 0.5 (female 1.2°/year and male 1.5°/year) and Risser 2 (female 1.1°/year and male 1.4°/year).When the Risser grade reached 3 which was a sign of the end of skeleton growth,the PI still had increasing potential and it increased slower in Risser 4 and Risser 5.There showed significant correlation between △PI and △age in the three group(P＜0.05)and the correlation was stronger in Low Risser group and Moderate Risser group than that in High Risser group.There showed significant correlation between △PW and △PI in all groups while △PH and △PI showed correlation only in Low Risser group.There showed significant correlation between △SW and △PI,△FH-S and △PI,△S-C length and △PI (P＜0.05) except △S-C length in male Low Risser group and △FH-S in female High Risser group and the Moderate Risser group had the strongest correlation.The △S-C distance and △S-C ratio didn't show any correlation with △PI (P＜0.05) expect the male of High Risser group.Conclusion Pelvic incidence is increasing during skeleton growth at the age of 9 to 17.It has the peak growth velocity in Risser 1 and remains growth potential in Risser 5.PI growth is correlated with pelvic height,femoral head-sacrum distance,pelvic width and sacral width.