SUMMARYWhile the use of a reference phantom is essential for dosimetry in acceptance testing and in regular quality control checks of a mammographic X‐ray unit, it is also of importance to be able to estimate the patient dose in each individual investigation.Radiographic and physical data were analysed for a total of 212 women who between them were screened at three locations participating in a breast screening programme. The radiologists made estimates of the individual breast composition (% glandular/adipose ratio) at the film reporting sessions, and then the glandular doses were calculated by the auditor according to the NCRP 85 methodology.Arising from the data analysis of this dosimetry survey, a method is proposed to determine objectively patient breast composition from the photo‐timed mAs for a given film optical density setting. This permits the NCRP calculations to be extended from breasts of ‘average’(50/50) composition to breasts of individually determined composition.The diversity of the results between the three locations emphasises the need for regular audits of a mammographic X‐ray unit's performance by an experienced radiological physicist, at least annually or after any major interventional service on