Aim: To explore factors predicting acquisition and loss of best walking ability in young people with bilateral cerebral palsy (CP). Method: In our population cohort (Study of Hips And Physical Experience) of 338 children (201 males, 137 females) with bilateral CP, age at achieving walking was recorded and walking ability predicted from early motor milestones. Walking was assessed at 5 to 8 years (mean 7y) and in 228 of 278 survivors at 13 to 19 years (mean 16y). Parent carers reported their view of any loss of best achieved walking. Factors potentially associated with loss of best achieved walking were explored: severity and type of motor disorder; intellect and communication; manipulative skill; general health and comorbidity; pain; orthopaedic surgery; musculoskeletal spine and lower limb deformity; weight; fatigue; mood; and presence of regular exercise regime. Results: The ability to walk independently was reliably predicted by the motor milestone ‘getting to sit and maintain sitting’ by the age of 36 months (without aids) and 55 months (with aids). Forty-five per cent of the cohort never walked 10 steps independently. Not all who achieved walking without aids were still doing so by a mean age of 16 years, which was associated with later age at achieving walking and the degree of musculoskeletal deformity, as was the parent carers’ report of loss of best walking. Interpretation: In this study, development of musculoskeletal deformity was a significant factor in not maintaining best achieved walking by mean age 16 years, which is most likely to occur in young people whose walking ability is with aids over short distances or in therapy only. Prediction of future walking ability in a child with bilateral CP can be made from early motor milestones.