The aim of this study was to determine whether reporting guidelines and computerised form-based reports improve the completeness of histopathological cancer data available for patient management and population cancer registration and to evaluate the acceptability of the intervention. The study was a randomised controlled trial with a split unit design and stratified cluster randomisation. All 16 hospital pathology laboratories in Wales were randomly allocated to report either breast or colorectal resection specimens by computerised form or conventional free text. 1044 reports were analysed in the study arm, 998 in the control arm. Use of pre-defined forms led to a 28.4% (95% confidence interval (CI): 15.7-41.2%) increase in complete reporting of a minimum dataset required for cancer registration and a 24.5% (95% CI: 11.0-38.0%) increase in complete reporting of minimum data required for patient management. Form-based reporting was acceptable to pathologists and preferred by clinicians. In conclusion, guidelines and computerised forms significantly improve the quality of histopathology reporting.