This article reports on the development of a research attitude in secondary education teachers who are conducting teacher research. We aimed to investigate the professional development of teachers who are conducting teacher research, focusing on the development of their research attitude. We operationalised the concept "research attitude", constructed a questionnaire and handed out questionnaires in which teachers self-reported on their research attitude. The study had a pre-test-post-test design. Questionnaire outcomes were compared with a control group of teachers. Findings show that the research attitude of the teacher-researchers increased during the school year, but the difference with the beginning of the school year was not significant. Compared to the control group, the teacher-researchers had a significantly higher research attitude at the end of the school year.