Early diagnosis of cancer is essential for the choice of appropriate therapy, and thus the chance of recovery. Parents play a crucial role in detecting the first signs of cancer in their children. The aim of this study was to assess parental knowledge on the early signs of leukaemia and to increase their cancer vigilance. A survey was conducted among 327 parents of 3–10-year-olds in two Polish towns: Nowa Sól and Nowe Miasteczko as well as on the Internet. Parents were asked about their responses to potential signs of leukaemia. The obtained data were analysed, considering parental education and gender, child’s health status and place of residence. A total of 98% of respondents were able to provide a correct definition of leukaemia. Enlarged lymph nodes, fever, cough, headache and vomiting are the most common reasons for medical appointments. Pale skin, fatigue and apathy were a group of symptoms that did not raise parental concern. Up to three-quarters of parents associated limb pain with the process of growing, and only 7% – with cancer. It was found that parents with at most secondary education reported to the doctor due to alarming symptoms more often than those with higher education. Women were more perceptive of suspicious symptoms than men. Parents of chronically ill children are more perceptive of extravasations, fever and bruises. Although leukaemia is not a taboo, its potential symptoms do not raise parental oncological vigilance. Women, parents of chronically ill children, parents without higher education and medical graduates show the highest vigilance in childcare. The presented data indicate the need for parental education on the first signs of cancer.