A novel serological assay which measures IgG(T) specific for a 12/13 kDa protein of the equine tapeworm Anoplocephala perfoliata was used as part of a colic outbreak investigation. A training/rehabilitation yard for Thoroughbreds and Arabs was found to have an increasing incidence of colic over a 5 year period, culminating in a peak incidence of 1.15 episodes/horse year at risk. Four animals suffered from ileal impaction colic which necessitated surgical management. A case-control study design suggested a strong association between tapeworm infection and colic, with evidence of a dose-response relationship. Intervention, in the form of anticestode anthelminthics, coincided with a decrease in the incidence of colic and a fall in anti-12/13 kDa IgG(T) antibody levels of 8 horses monitored post-treatment. This study demonstrates that anthelminthic regimens, using exclusively ivermectin, may lead to an increase in tapeworm infection intensity which may in turn lead to an increased incidence of colic. Furthermore, it provides support to the hypothesis that the risk of ileal impaction colic and spasmodic colic increases with tapeworm infection intensity. The practical application of the anti-12/13 kDa IgG(T) ELISA is demonstrated by this study.