Perianal disease occurs in up to 90% of patients with Crohn's disease [1-4]. Many of these patients have only mild symptoms or are asymptomatic and thus require no intervention. Clinical features are variable and include hypertrophic skin tags, ulceration, perianal abscess and fistulae, anal canal ulcers, fissures, induration and stenosis. Perianal abscess and fistula often occur simultaneously and are usually symptomatic. Symptoms range from pain, discharge, bleeding, to gross faecal incontinence with restriction of lifestyle and sexual activity. There is little uniformity amongst clinicians in the investigation and management of perianal Crohn's disease [5]. This is due, in part, to the variability in both frequency and severity of attacks and to spontaneous remissions and exacerbations of perianal disease. Secondly, assessment of severity of illness and the response to treatment is difficult to objectively quantitative. Improvement in quality of life is the aim of therapy not cure of perianal disease. Investigative modalities for perianal Crohn's are changing due to the limitations of conventional fistulography, CT scanning and clinical evaluation. MRI scanning has been introduced more recently, however, requires an endorectal coil to obtain good anatomical visualisation and has limited availability [6-12]. Endorectal ultrasonography has been shown to detect more abscesses and fistula in Crohn's patients than clinical examination, proctosigmoidoscopy and CT scanning, better delineation of fistulous tracts than fistulography and has the ability to change the clinical management of referring physicians [13-16]. Most fistulae are not explored surgically and therefore the documentation of fistulae in symptomatic Crohn's disease has been limited and are usually classified only as high or low [4]. Park's has pointed out this terminology for cryptoglandular disease is "... an ambiguous one" and hence developed a more precise nomenclature [17]. The objective of this study was to document prospectively by transanorectal ultrasonography fistulae and abscesses in symptomatic perianal Crohn's disease and to classify them according to Park's nomenclature and determine the incidence of these at the time of referral for a new exacerbation of the disease. Anal wall thickness was measured prospectively by ultrasonography as it has been shown to be increased in patients with perianal Crohn's disease and may reflect disease activity [13, 18].