A review of closure of Hartmann's colostomy was undertaken to establish guidelines for the timing and technique of reversal. Between 1984 and 1990 there were 69 reversals; 48 patients originally had diverticular disease and 21 had carcinoma. One-third underwent reversal before 4 months and two-thirds after this time. The operative mortality rate was 3 per cent and the anastomotic leak rate 4 per cent. Significant morbidity occurred in 30 per cent. There was no advantage in delayed closure. Complications occurred in 24 per cent of patients undergoing reversal before and 35 per cent undergoing reversal after 4 months. Thirty-five anastomoses were hand-sewn and 34 stapled. There were no differences in operating time for the two techniques, but a greater number were stapled after 4 months than before (P less than 0.05), which may reflect increased rectal stump shrinkage with time. There were no differences in complication rates whether the anastomosis was hand-sewn (34 per cent) or stapled (26 per cent). Closure of Hartmann's colostomy is a safe procedure but has a significant morbidity in nearly one-third of cases. On the basis of these results, there is no indication to delay closure after 4 months have elapsed, and earlier reversal, when the rectal stump is most accessible, is recommended.