Perianal fistulas can occur to up to one-third of patients with Crohn's disease (CD) leading to significant disabling disease and morbidity. Fistulising perianal CD treatment often necessitates a combined pharmacological and surgical approach. Anti-tumor necrosis factor (anti-TNF) therapy, particularly infliximab, has been shown to be very effective for both perianal and internal fistulising CD. Nevertheless, current data suggest that sustained remission and long-term complete fistula healing can be achieved in only 30% to 50% of patients. Moreover, these percentages refer mostly to clinical rather than deep remission, defined as endoscopic and radiologic remission, which is quickly emerging as the preferred goal of therapy. Unfortunately, the therapeutic options for perianal fistulising CD are still limited. As such, it would be of great value to be able to predict, and more importantly, prevent treatment failure in these patients by early and continued optimization of anti-TNF therapy. Similar to ulcerative colitis and luminal CD, recent data demonstrate that higher infliximab concentrations are associated with better clinical outcomes in patients with perianal fistulising CD. This suggests that therapeutic drug monitoring and a treat-to-trough therapeutic approach may emerge as the new standard of care for optimizing anti-TNF therapy in patients with perianal fistulising CD.