The importance of numeracy for professional practice in nursing is increasingly recognized in the USA, UK and elsewhere, but there is no consensus on what is meant by numeracy for nursing, nor on how it should be taught, learned and assessed. Meanwhile, studies in various countries indicate a lack of proficiency in numeracy of some students and qualified nurses. Against this background, the authors undertake an exploratory interdisciplinary (nursing and education) study to evaluate the teaching, learning and assessment of numeracy for nursing in a large pre-qualification program at a school of nursing in England and draw out implications for the teaching, learning and assessment of numeracy for nursing in the school and elsewhere. The authors' analysis points up the dangers of high stakes testing with a 100% pass mark in the absence of a reliable and valid assessment instrument set to an agreed standard and reflecting the scope of numeracy for nursing. While this is intended to indicate mastery of a safety-critical aspect of nursing practice, the analysis shows that the test in the study does not warrant this. The test is neither reliable nor valid; it does not consistently test what it is intended to test. (Contains 3 figures.)