The aim of this study was to evaluate the dental treatments under nitrous oxide-oxygen sedation carried out during 1 yr by the first 45 Swedish dentists trained at probationary courses in the use of the technique. Special emphasis was placed on evaluating the risk and incidence of side effects. Data from 1719 treatment sessions in 823 patients, mainly children, were analyzed. Standardized sedation technique was used and the maximum level of nitrous oxide administered was set at 60%. About 90% of the patients showed excellent or fair acceptance. Factors influencing the acceptance were the patient's age, history of psychiatric disorders, mental retardation and occurrence of side effects. In 4.5% of the treatment sessions the patient experienced side effects, e.g. restlessness, vomiting or nausea, during treatment and in 0.9% after the treatment session. The side effects were mainly mild. No correlation was found between side effects and the nitrous oxide concentration used, length of treatment, patient's age or health classification. It is concluded that nitrous oxide-oxygen sedation is an excellent and safe aid to dental care.