Emergence data on the 20 deciduous teeth and the first 29 permanent teeth were collected from 212 randomly selected urban Swedish children who were followed from birth to 18 years of age. The sex difference in the emergence of the deciduous teeth is less than one month, which is not statistically significant. Boys are consistently ahead of girls until the 17th deciduous tooth. From the 17th deciduous tooth on through most of the permanent dentition, girls are consistently ahead of boys. In the permanent dentition the sex difference ranges from 3 MO to 11 MO; these differences are statistically significant except for the 29th tooth. Reference data on dental age based on counts of 1-19 deciduous and 1-27 permanent teeth are tabulated and tooth emergence curves constructed. The tooth emergence curves can be used to express individual dental development in terms of standard deviation scores. Validity of dental age assessed by counts of permanent teeth is evaluated by a cross-sectional comparison with another sample of Swedish boys and girls. The mean difference between estimated age and chronological age is about one month in either sex. Precision of an individual estimate of dental development in terms of 95% confidence level (approximately equal to +/- 2 S.D.) varies from about +/- 4 months in the deciduous dentition to +/- 3 years in the permanent dentition. Assessment of dental development and dental age by means of tooth counts is a convenient and simple method, although it can only be applied at ages when emergence can be expected. It is especially useful in cross-sectional evaluations, as no serial data are required. In populations with a low incidence of caries the impact of such disturbing factors on emergence is correspondingly low, further increasing the validity of assessments of dental development based on tooth emergence.