Abstract An experiment is presented which shows that colonic primary tumours cluster around a previously constructed anastomosis and that the choice of suture materials (but not the technique of their insertion) can influence this. In a rodent model where dimethylhydrazine was administered 2 months after surgery, anastomotic tumours were most often seen when wire sutures — as used in the staples of anastomotic stapling guns — had been employed (10 out of 16 large bowel tumours were anastomotic as compared with 2 of 12 in a silk sutured group, P = 0·019). An explanation may be that wire sutures persist much longer than silk (in the experiment, 10 months after insertion, 4 per cent of silk sutures were still present, 15 per cent of wire, P < 0·01). This was translated into a greater degree of scarring at the anastomosis, being most severe in the presence of persisting sutures. Of the 12 anastomotic tumours found in both groups, 7 (58 per cent) were in the minority (17–26 per cent) who had persisting sutures and the remaining 5 in the 47 who had none. Techniques of suturing (needle always passed from the serosa in; needle from mucosa out — the latter in such a way that mucosal cells could be displaced into the bowel wall where it was supposed that they might be more susceptible to subsequent carcinogenesis) did not affect tumour yield. We suggest that non-absorbable sutures, and especially stainless steel wire, should not be used when constructing an anastomosis after large bowel cancer surgery.