Researchers have long appreciated the role of climate in vector-borne diseases, including the resurgence of boutonneuse fever (BF). Portugal usually is classified as having temperate Mediterranean climate. In this new century, in analyzing the data from the Meteorology Institute, this pattern has changed and an accentuated variability in climate is being observed. BF (febre escaro nodular) is endemic and high season is from late spring and summer. The brown dog tick Rhipicephalus sanguineus. is the vector and reservoir of Rickettsia conorii complex strains: R. conorii Malish and Israeli spotted fever strain. To assess the influence of climate change in BF seasonality our aim was to compare the human sera samples received at CEVDI-INSA for laboratory diagnosis of MSF for 5 months per year from October to February, ("off-season") from 2000 to 2005. Of 1,299 sera samples in persons with suspected clinical diagnosis of MSF, 45 (3.4%) were considered positive cases and the number of positive cases has doubled in the last 2 years. BF epidemiology clearly appears to be associated with climate change, especially with low precipitation values. Physicians should be aware of increasing off-season BF cases.