['Background Anal cancer is increasing in incidence, has very high rates in specific populations and shares many similarities with cervical cancer. High-grade squamous intraepithelial lesions (HSIL) are regarded as precursors to anal cancer. High resolution anoscopy (HRA), which is derived from colposcopy, is the only currently available tool that can identify areas of the anal canal for targeted biopsy and identification of HSIL.', 'This study investigated the ability over a period of time of a single anoscopist to identify and adequately biopsy HSIL, correlating with contemporary anal cytological findings.', 'Four hundred paired cytology and histology samples collected from 283 patients over a 7-year period from 2004 to 2010 were compared. There was a significant increase in HSIL detection rates when anal squamous cells of undetermined significance (ASC-US; 38.6-66.0%) or low-grade squamous intra-epithelial lesion (38.8-68.3%) were taken as cut-off points (P<0.001 for both). Detection rates did not change significantly when atypical squamous cells-cannot exclude HSIL (ASC-H) or a higher grade lesion (70-76.6%) was taken as the cut-off point.'] Does ability to detect high-grade squamous anal intraepithelial lesions at high resolution anoscopy improve over time?