The objective of this study was to examine factors affecting morbidity after tonsillectomy in children. Data from the National Tonsil Surgery Register in Sweden on 18,712 patients who underwent tonsillectomy with or without simultaneous adenoidectomy between 1 and 18 years of age were analysed. This register includes data on sex, gender, surgical indication, and the surgical and haemostasis techniques used for each patient, as well as patient-reported outcomes for haemorrhage, analgesic use and antibiotic use. Comparison of patients who underwent surgery for infection versus upper airway obstruction revealed a significant increase in haemorrhage complications in the infection group. However, no significant difference remained after the adjustments for confounders in multivariable regression analysis. Instead, the increased risk among patients who underwent surgery for infection was mainly attributable to the use of bipolar diathermy and increased patient age. Patients who received surgery for infection reported more days of analgesic use, as well as more unplanned contacts with a health care service provider due to pain, compared with those who underwent surgery for upper airway obstruction. These results remained significant in multivariate analysis. The use of bipolar diathermy for haemostasis resulted in an increased risk, while the use of cold steel surgical instruments, a younger patient age and female sex led to a decreased risk. The surgical and haemostasis techniques used are the most important factors that affect morbidity after tonsillectomy in the paediatric age group. The choice of surgical techniques is of utmost importance for decreasing morbidity in these patients.