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The declining or fluctuating trend in blood pressure (BP) despite the rising trend in body mass index (BMI) during childhood and adolescence is unexplained. We decomposed trends in BP and BMI to identify the relevance of early-life and contemporaneous factors.We assessed the relative contribution of age, period and cohort to secular trends in BP in children and adolescents (9-18 years) from 1999 to 2014 and BMI (6-18 years) from 1996 to 2014 in Hong Kong, China.After accounting for age, period effects contributed more than cohort effects to the overall fluctuating BP trend and the rising BMI trend observed in this Chinese population. For both sexes, BP fell from the start of period to a low point in 2003-2005 but then rose. BMI rose strongly across the period before levelling off in 2009-2010. Earlier cohorts (born in 1983-1984) had higher BP and BMI than later cohorts.With globalisation and associated lifestyle changes, successive generations of children and adolescents in a recently developed Chinese setting had lower BP and BMI, but this fall was offset until recently by population-wide increase in BMI. School-based health promotion efforts could have partly mitigated the population-wide rise in child and adolescent BMI, while socioeconomic transition or other factors could be relevant to changes in BP between generations. Explaining these trends will help identify early-life factors that may contribute to a healthier start as well as contemporaneous factors that may protect against rising trends in adiposity.