The rate of "interval" cancers, those diagnosed between mammograph examinations, is higher among women aged 50-59 compared to those 60-64. The menopause typically occurs between the ages of 45 and 50 and this is when women are most likely to try HRT.
Writing in the Lancet,epidemiologists from the Imperial Cancer Research Fund's unit in Oxford believe there is probably a link with HRT. In a letter, Valerie Beral and her colleagues say HRT at the time of screening increases the density of breast tissue, as seen by a mammograph X-ray. This may lower the chance of women having cancers detected in their earliest stages by screening.
A US study published last year showed the relative risk of having an interval cancer diagnosed as opposed to a screen-detected cancer was four times higher for women on HRT. The results suggested that in the UK in 1990 about 700 extra interval cancers would have been diagnosed among HRT users.
"This excess is sufficient to account for the higher than expected rate of interval cancer in the NHS screening programme as a whole," wrote Dr Beral.
She urged caution. "We do not know whether this will turn out to be the case. There is simply not enough evidence yet to know whether more cancers are being missed among women taking HRT.
"If research shows HRT does reduce the efficacy of screening among current users and if it also shows efficacy is restored once women stop taking it, a simple way to reduce interval cancers may be to suggest that a woman stops using HRT for a short time, perhaps some weeks or months, before she is due to be screened."