Objectives To examine associations of gain in body mass index (BMI) and linear growth from term to 18 months with asthma at age 8 years in a cohort of preterm infants. We hypothesised that rapid BMI gain would increase asthma risk, whereas rapid linear growth would be protective.

Design Observational secondary analysis of data from the Infant Health and Development Program.

Setting 8 centres in the USA.

Patients 863 preterm (≤37 weeks), low birthweight (≤2500 g) children.

Main exposure variables Gain in BMI and linear growth from term to 4 months, 4–12 months and 12–18 months, in z-scores.

Main outcome measure Asthma at age 8 years, assessed by parent report of the child ever receiving the diagnosis from a doctor.

Results At age 8 years, 149 (17%) had ever been diagnosed with asthma. Adjusting for maternal and child factors in logistic regression, for each additional z-score gain in BMI from term to 4 months, odds of asthma at age 8 years were higher (OR 1.3, 95% CI 1.1 to 1.5); findings were similar for BMI gain from 4–12 months. More rapid linear growth from term to 4 months was not associated with lower odds of asthma (OR 1.1, 95% CI 0.9 to 1.3), with similar findings for 4–12 months and 12–18 months.

Conclusions More rapid BMI gain in the 1st year of life was associated with higher odds of asthma, whereas linear growth did not appear protective. Our results add to mounting evidence that excess weight gain after term may be harmful to preterm infants.