Using data from seven malformation monitoring systems around the world, the feasibility of monitoring fresh dominant mutations using skeletal dysplasias was explored. Based on a total of over 9.5 million births, 1500 infants with skeletal dysplasias were identified (16 per 100,000). In spite of efforts to get exact diagnoses, an average of 21% were unspecified. Comparisons of rates of specific diagnoses in different programmes suggested that classification differed. By analysing maternal age distribution, estimates were made of the proportion of fresh mutations in different subgroups: conditions regarded as dominant (achondroplasia, thanatophoric dysplasia, spondyloepiphyseal dysplasia) were estimated to consist of 58% fresh mutations--some of the remaining cases were inherited, others were probably misclassified. Among conditions regarded as recessive, only 5% were estimated to be truly dominant mutations. In the total group of skeletal dysplasias, 21% were estimated to be fresh dominant mutations and if osteogenesis imperfecta were excluded, the figure was 31%. By power analyses it was shown that equal monitoring power may be obtained by a programme covering about 45,000 births per year with intensive diagnosis of each individual case of skeletal dysplasia and a programme some three times greater where no specific diagnoses are obtained. An increasing trend in the occurrence of skeletal dysplasias was seen but probably explained by changing ascertainment. An impact of antenatal diagnosis resulting in a decrease in occurrence was also apparent in some programmes.