Incisional hernia is the most frequent long-term complication after visceral surgery, with an incidence of between 9 and 20 per cent 1 year after operation. Most controlled studies provide only short-term follow-up, and the actual incidence remains unclear. This study evaluated the incidence of incisional hernia up to 3 years after midline laparotomy in two prospective trials. Three-year follow-up data from the ISSAAC (prospective, multicentre, historically controlled) and INSECT (randomized, controlled, multicentre) trials focused on the rate of incisional hernia 1 and 3 years after surgery. Differences between the two groups were compared using t tests for continuous data and the χ2 test for categorical data. Analysis of 775 patients included in the two trials suggested that the incisional hernia rate increased significantly from 12.6 per cent at 1 year to 22.4 per cent 3 years after surgery (P < 0.001), a relative increase of more than 60 per cent. This follow-up of two trials demonstrated that 1 year of clinical follow-up for detection of incisional hernia is not sufficient; follow-up for at least 3 years should be mandatory in any study evaluating the rate of postoperative incisional hernia after midline laparotomy. © 2013 BJS Society Ltd. Published by John Wiley & Sons Ltd.