This study describes the self-assessed activities, features, prerequisites, and consequences of occupational health nurses' expertise. The quantitative data were gathered from 468 Finnish occupational health nurses, of whom 373 (80%) returned the completed questionnaire. The data were analyzed by using one-way-anova, the Kruskal-Wallis or the chi(2)-tests. The activities of the occupational health nurses included working with employees, workplaces, and collaborative partners, administrative and office work, and other duties. The most important expert features were the holistic perspective and listening to clients. Continuing training and a positive attitude were the most necessary prerequisites for expertise. The main benefits of expertise, from the perspective of the occupational health nurses, were improved health and a decreased number of work-related health risks. Support by the work community and good educational possibilities were important for expert practice. Occupational health nurses need to develop their expertise continually because they play a key role in promoting workers' health.