The aim of this study was to establish whether nitrous oxide has a clinically important effect on postoperative vomiting in children after myringotomy. We studied 320 healthy children of ages 0.5-13 yr undergoing elective myringotomy and tube insertion. Induction and maintenance of anaesthesia were randomized to inhalation with either 70% N2O/30% O2/halothane or 100%O2/halothane. Surgical technique and postoperative management were not influenced by this study. Vomiting in the recovery room (PAR) and Day Care Surgical Unit (DCSU) was recorded by nurses unaware of the anaesthetic technique. Parents were contacted 24-48 hr after surgery to ascertain the incidence of vomiting after discharge. The groups were similar with respect to demographic data, except that the anaesthesia time was greater among the 158 patients in the N2O-treated group (11 +/- 4 vs 12 +/- 4 min, mean +/- SD). The incidence of vomiting was 13% in both groups. Most of the 42 patients that had emesis only vomited once or twice. The incidence of vomiting was not altered by sex (13% vs 13%) or duration of anaesthesia. The incidence of vomiting increased with increasing age. The children aged less than 3 yr vomited 4% of the time, those aged 3-5 vomited 11% of the time, those aged 6-8 yr vomited 17% of the time, while the incidence of vomiting among those aged 9-13 yr was 31%. Vomiting prolonged the postoperative hospital stay from 75 to 92 min, P < 0.001, ANOVA. In summary, we have been unable to demonstrate that N2O induces vomiting by children after a brief general anaesthetic for myringotomy.