The National Breast Screening Service in the UK depends upon triple assessment, with or without core biopsy, to determine the need for diagnostic biopsy or treatment for screen-detected lesions. To determine the efficacy of cytology in this process a nationwide survey of sensitivity and specificity of the technique was instituted by the National Coordinating Committees for both Radiology and Pathology. Although some units were performing to the national recommended standards, a significant number of units were shown to have problems with a high false negative rate and high inadequate rates from lesions which were eventually diagnosed as cancer. This suggests problems in localizing lesions adequately at aspiration. Generally, indicators of pathology diagnostic performance (e.g. positive predictive values) were good. The figures suggest that some units are not benefiting from an optimal cytological service. Consequently training programmes targeted at aspiration technique may be beneficial.