The healing of midline laparotomy incisions closed with a continuous suture of nylon or second-generation polydioxanone was evaluated in a randomized clinical trial. The effect of suture technique, reflected in the suture length to wound length ratio, was also assessed. All patients who underwent abdominal surgery through a midline incision were included except those with incisional hernia after previous midline operation. Wound dehiscence occurred in five (0·6 per cent) of 813 patients and wound infection in 73 (9·0 per cent). These rates were similar for both suture materials, as were those for the development of suture sinus and prolonged postoperative wound pain. Incisional hernia 12 months after surgery was found in 49 (15·1 per cent) of 325 wounds sutured with polydioxanone and in 50 (15·7 per cent) of 318 closed with nylon (P=0·91). There was a significant correlation between the hernia rate and the suture to wound length ratio for both materials (P<0·001). These results indicate that suture of midline laparotomy wounds is as safe with polydioxanone as it is with nylon. Incisional hernia is associated more with suture technique than with the material used.