Background. The current understanding on caries and enamel developmental defects prevalence and frequency, caries risk, health-related quality life (HRQoL) and stress response in young children with cleft lip and/or palate (CL/P) are sparse. In this thesis these aspects were investigated in 5- and 10- year-old children with CL/P in comparison to non-cleft children in the same ages. Design. The studies in this thesis have a cross-sectional case-control design. Participants. The study group consisted of 139 children with CL/P (80 children aged 5 years and 59 aged 10 years) and 313 non-cleft controls (144 children aged 5 years and 169 aged 10 years). Method. Caries was scored according to International Caries Detection and Assessment System (ICDAS II) and developmental enamel defects were measured as presence and frequency of hypoplasia and hypomineralization. Oral hygiene was assessed using Quigley-Hein plaque index. Stimulated saliva samples were analyzed for mutans streptococci, lactobacilli, buffering capacity and secretion rates. Information regarding children’s oral hygiene routines, dietary habits and fluoride exposure were collected with questionnaires. Caries risk was evaluated with algorithm-based software, Cariogram while HRQoL was perceived with KIDSCREEN-52. Stress response was analyzed with cortisol concentration in saliva at three different time points using a commercial competitive radioimmunoassay. Results. Caries prevalence (36% versus 18%) and caries frequency (1.2 dmfs versus 0.9 dmfs) was significantly higher in 5-year-old children with CL/P in comparison to non-cleft controls. In 10-yearolds no significant difference was found between children with CL/P and non-cleft controls in caries prevalence (47% versus 38%) or in caries frequency (0.7 DMFS versus 0.5 DMFS). Children with CL/P had significantly higher prevalence of enamel defects, higher counts of salivary lactobacilli and less good oral hygiene. The odds of being categorized with high caries risk were elevated in children with CL/P. Children with CL/P had similar HRQoL and salivary cortisol concentrations as non-cleft controls. However, 10-year-old boys with CL/P had significantly higher cortisol concentrations in the evening than non-cleft boys. Conclusions. Preschool children with CL/P seem to have more caries in the primary dentition than non-cleft controls. Children with CL/P had increased odds of being categorized as high caries risk individuals compared to controls. Some of the contributing factors seem to be higher prevalence of enamel defects, impaired oral hygiene and elevated salivary lactobacilli. Furthermore, as measured with the help of cortisol concentrations in saliva, children with CL/P were not more stressed than noncleft controls and their HRQoL was comparable to a European norm population. It appears that regular comprehensive preventive oral care in children with CL/P is effective in preventing caries development in permanent teeth. However, children with CL/P are at risk of caries development and preventive oral care should be implemented and started earlier than today.