The fact that we did not observe an increased HF rate in patients treated with both aromatase inhibitors and tamoxifen may be explained by treatment duration; upfront aromatase inhibitors were generally prescribed for a duration of 5 years compared to a duration of 2.5 years when combined with tamoxifen (Dutch treatment guidelines, http://www.oncoline.nl). Here, CYP19A1 is linked to hydrops fetalis.