Troublesome perianal disease occurs in ~35% of patients with Crohn's disease, yet conventional disease activity indices do not reflect the severity of this feature. To assess the degree of impairment and response to therapy, we identified five simple elements and graded each on a 5-point Likert scale in 37 patients at 124 visits. At each visit a Crohn's Disease (CDAI) or Simple Activity Index (HBDAI), Perianal Disease Activity Index (PDAI), and treatment were recorded. The PDAI was validated against physician (MDGA) and patient (PGA) global assessments, and treatment was prescribed for the perianal disease. Measurement error was evaluated in 19 patients who were clinically stable at two consecutive visits. The ability of the PDAI to detect important clinical change was tested in 20 subjects exhibiting a change on PGA at consecutive visits. There were strong correlations between PDAI, MDGA, and PGA scores at all visits (R=0.66-0.72; p<0.001), whereas the CDAI and HBDAI correlated poorly with PDAI (R<0.23). Physicians prescribed more aggressive therapy for higher PDAI scores (r=0.53). Mean PDAI scores between visits in clinically stable subjects were not significantly different [5.58 ± 2.79 (initial); 5.42 ± 2.55 (follow-up); p=0.63]. PDAI significantly improved between visits when the perianal disease had improved (PDAI score difference 3.05 ± 2.96; P=.0002). We conclude that the PDAI is simple and clinically useful for patient management. It should now be assessed in a clinical trial.