The development of the dental arches in children with oral clefts differs from that in a normal population, due to the type and extension of the cleft, surgical procedure and timing, and decreased growth potential. The size of the maxillary and mandibular dental arches and the amount of interdental space in 3-year-old, cleft-affected and non-cleft children was investigated. Fifty non-cleft (NONC) and 104 cleft-affected subjects including different cleft groups were compared. On average, cleft lip was corrected at the age of 0.6 years and cleft palate at 1.8 years of age. The mean of all width and depth dimensions in the cleft lip group were close to the NONC controls, whereas the dimensions of the cleft palate group were 8-9 per cent smaller in the maxilla and 5-7 per cent smaller in the mandible than were those in the NONC group. In the unilateral complete cleft group, the maxillary dimensions were 11-19 per cent smaller, but in the mandible only 0-4 per cent smaller than in the NONC group. In the bilateral complete cleft group, the maxillary arch was only 6 per cent shorter but 30 per cent narrower at the canines than in the NONC group. In the mandible the corresponding differences were 2 per cent and 6 per cent. Forty per cent of the cleft palate subjects had a crowded maxillary arch compared with 6 per cent of the NONC controls.(ABSTRACT TRUNCATED AT 250 WORDS)