The aim of this cross-sectional clinical outcome study using retrospective data capture of treatment histories was to examine the characteristics of children born with unilateral cleft lip and palate (UCLP) in the United Kingdom (UK) who were not grafted at the appropriate age or who had an unsuccessful secondary alveolar bone graft. The subjects were born with complete non-syndromic UCLP between 1.4.82 and 31.3.84 and were aged between 12.0 years and 14.7 years at the time of data collection under the care of 48 cleft teams. The success of secondary alveolar bone grafting was assessed using a modification of the Bergland index. There were no independent predictors for unmet bone grafting need. The outcome of secondary alveolar bone grafting was assessed for 164 subjects; 90 (55%) had a successful first graft. Non-Caucasian (P = 0.037) and increasing age at grafting (P = 0.007) were risk factors for poor outcome. After adjustment for other risk factors, increased age at grafting was independently associated with having a seriously deficient or failed graft (OR = 1.03; 95% CI 1.01-1.06 P = 0.036). All the non-Caucasians in this sample had an unsatisfactory graft. Increasing age in months at grafting and ethnicity are predictors for poor outcome of secondary alveolar bone grafting in children born with UCLP in the UK.