BACKGROUND Preterm infants are recognised as developing at a significantly slower rate than their full-term peers and with different movement quality. OBJECTIVE This study aimed to describe the longitudinal gross motor trajectories of these infants in the first 18 months of (corrected) age and investigate factors associated with gross motor development. METHODS A longitudinal study was conducted with convenience samples of 58 preterm infants born < or = 29 weeks of gestation and 52 control full-term infants in Australia. METHODS The infants were assessed at 4, 8, 12 and 18 months of (corrected) age using the Alberta Infant Motor Scale (AIMS). RESULTS Forty-six preterm and 48 control infants completed all four assessments. The preterm group scored significantly lower on various sub-scores at all age levels. Almost half of the preterm infants demonstrated less progression in the sit sub-scale from 4 to 8 months (corrected) age, possibly due to an imbalance between flexor and extensor strength in the trunk. At 12 and 18 months of (corrected) age, lack of rotation and fluency in their movements were evident in some preterm infants. Presence of intra-ventricular haemorrhage and chronic lung disease were associated with poor motor performance at 4 months and use of postnatal steroids was associated with poor motor performance at 4, 8 and 18 months of corrected age. CONCLUSIONS The imbalance between flexor and extensor muscle strength in preterm infants had a stronger impact on motor development than usually expected. The AIMS appears to be a sensitive assessment tool to demonstrate the unique movement characteristics in this preterm cohort.