Background. Different surgical variables are assumed to play a role in postoperative course after lower third molar extraction. The aim of study was to assess whether flap design and duration of surgery can influence acute postoperative symptoms and signs after lower third molar extraction. Methods. Twenty-five patients scheduled for lower third molar extraction were included in this study and randomly assigned to two groups in terms of flap design: group A (envelope flap) and group B (triangular flap). Swelling and trismus were assessed before and after surgery on days 0, 2 and 7. Pain was assessed for seven days after surgery. Maximum postoperative pain was chosen as the main outcome variable. ANOVA was used to assess differences between the groups regarding maximum postoperative pain, trismus and swelling at 2- and 7-day intervals. Pearson's correlation coefficient was used to assess correlation between duration of surgery and postoperative symptoms and signs. Results. No significant difference was found between the two flap designs for any postoperative symptoms and signs. The duration of surgery was found to be correlated with both trismus (r = -0.44, P = 0.04) and swelling (r = 0.59, P = 0.004) as assessed 2 days after surgery. No associations were found between duration of surgery and maximum postoperative pain and trismus and swelling at 7-day interval. Conclusion. Within the limits of the present study, the duration of surgery, and not the flap design, affected the acute postoperative symptoms and signs after lower third molar extraction.