Written lesson appraisals (WLAs) by mentors (n = 30) of pre-service teachers experiencing two different mentoring regimes in an English university teacher education programme were selected for analysis. The WLAs were analysed for their length and content using professional knowledge categories derived from pre-service teacher perceptions of the content of WLAs. The professional knowledge categories were "topic-specific pedagogy", "class management" and "generic issues". Statistically significant differences were found in the length of WLAs written by mentors from the two mentoring regimes and in the content of all professional knowledge categories. Pre-service teacher "perceptions" of content are similar to the "actual" content of the WLAs but the degree of emphasis was different. Implications for the training and continuing professional development of mentors, pre-service teachers and others who receive or provide WLAs are identified. (Contains 3 tables and 1 figure.)