Context Overweight/obesity in children is increasing. Incidence data for medical complications use arbitrary cut-off values for categories of overweight and obesity. Continuous relationships are seldom reported.

Objectives To report relationships of child Body Mass Index (BMI) z-score as a continuous variable with the medical complications of overweight.

Design Part of the larger, prospective cohort Growth and Development Study.

Setting Community recruitment of children through randomly selected primary schools. Clinic recruitment of overweight treatment-seeking children through tertiary centers.

Participants Children aged 6-13 yr were community-recruited normal weight (n = 73), community-recruited overweight (n = 53) and overweight treatment-seeking (n = 51). Medical history, family history, and symptoms of complications of overweight were collected by interview and physical examination performed. Investigations included Oral Glucose Tolerance Tests (OGTT), fasting lipids and liver function tests.

Main Outcome Measure Adjusted regression was used to model each complication of obesity with age- and sex-specific child BMI z-scores entered as a continuous dependant variable.

Results Adjusted logistic regression showed the proportion of children with musculoskeletal pain, obstructive sleep apnoea symptoms, headaches, depression, anxiety and bullying, and acanthosis nigricans increased with child BMI z-score. Adjusted linear regression showed BMI z-score was significantly related to systolic and diastolic blood pressure, insulin during OGTT, total cholesterol, high density lipoprotein, triglycerides, and alanine aminotransferase.

Conclusion Child's BMI z-score is independently related to complications of overweight and obesity, in a linear or curvilinear fashion. Children's risks of most complications increase across the entire range of BMI values and are not defined by thresholds.