Six mature horses with large abdominal abscesses (defined as an abscess >15-20 cm in at least one dimension) were treated by surgical drainage and post operative lavage. The abscess was associated with previous intestinal surgery in one horse, and with Streptococcus equi spp. equi infection in the other 5. A Foley catheter was used to drain and lavage the abscess in all cases. The abscess was accessed by a ventral midline laparotomy in 5 cases and by standing flank laparoscopy in one. Two horses were subjected to euthanasia within 7 days due to persistent or recurrent colic. The other 4 horses survived. Lavage of the abscess was continued for a median time of 19 days. Antibacterial therapy was continued until the plasma fibrinogen concentration was normal (median 47 days). Follow-up information was obtained at a median of 1.8 years. All 4 horses were alive at the time of follow-up; 2 horses had suffered one or more bouts of colic that had responded to medical treatment.