Hay fever or pollinosis is the allergic reaction of the human body to allergic pollen grains and is a seasonal phenomenon. Pollen concentrations depend on the climate, geographic features, and vegetation. Trees, grass, and weed pollinosis is frequent in Croatia, common ragweed (Ambrosia artemisifolia L.) pollinosis in particular. Continuous monitoring of pollen air concentrations can provide timely information to the general public and can help sensitised patients and their physicians to prevent or alleviate allergic reactions. This is the task of health ecology services such as our Public Health Institute of the Brod-Posavina County. This article reports pollen concentrations in Slavonski Brod measured in March 2008 to November 2008, March 2009 to October 2009, and April 2010 to October 2010 and discusses the increasing exposure to ragweed and ways to control it.