Background: This article investigates the prevalence of conditions that affect cognitive and/or psychosocial functioning in 10-year-old children born with a cleft lip and/or palate (CL/P) and explores how the presence of such additional difficulties may affect the reporting of outcomes in psychological research. Design: Cross-sectional data derived from routine psychological assessments. Setting: Centralized treatment (country). Participants: Data on cleft type and additional conditions were collected for 754 children with CL/P from 11 consecutive birth cohorts. Data on psychological adjustment were collected for three consecutive birth cohorts (n = 169). Outcome Measures: The Strengths and Difficulties Questionnaire (SDQ), completed by children and parents. Results: A total of 240 children (32%) in the sample had an additional condition, such as developmental delay, attention deficit/hyperactivity disorder, or a specific language impairment or dyslexia. Analysis of SDQ scores using conventional exclusion criteria (approach 1) was compared with a second method (approach 2), which included all children and categorized them according to the presence or absence of additional conditions. Significant variation in profiles of psychosocial adjustment was found depending on the approach to exclusion. Conclusions: The presence of additional conditions in a sample may affect results and subsequently the conclusions drawn in relation to the psychosocial adjustment of children born with CL/P. The present study emphasizes the importance of careful assessments and reporting of all associated conditions, in order to improve the understanding of the impact of a cleft and the consequences of associated conditions in this population.