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To explore the developmental origins of cardiorespiratory fitness.We examined the associations of birth size and duration of breast feeding with cardiorespiratory fitness assessed at the 9 year follow-up examination in 3612 participants of the Avon Longitudinal Study of Parents and Children (ALSPAC). We used physical work capacity at a heart rate of 170 beats per minute (PWC170) as our assessment of cardiorespiratory fitness. This was estimated using standard regression methods from parameters measured using an electronically braked cycle ergometer.Birth weight, length and ponderal index were all positively associated with cardiorespiratory fitness in both sexes, with no strong evidence of a difference in effect between girls and boys. Work capacity increased by 1.12 W (95% CI: 0.83, 1.40) on average per 1 standard deviation (SD) greater birth weight. This association was not affected by adjustment for socioeconomic position and maternal smoking during pregnancy; there was some attenuation with adjustment for both maternal and paternal height and body mass index and more marked attenuation with adjustment for the child's height and body mass index. In the fully adjusted model work capacity increased by 0.51 W (95% CI: 0.21, 0.81) per SD birth weight. Whether an individual had been breastfed and duration of breastfeeding were not associated with cardiorespiratory fitness in any models.Our results provide some support for a role of intrauterine factors in determining cardiorespiratory fitness in childhood.