As for many cardiovascular events, pulmonary embolism (PE) is not randomly distributed over time, but shows rhythmic patterns. The purpose of this study was to investigate whether such temporal pattern of occurrence varied in subgroups of patients according to different risk comorbid conditions. All cases of PE observed at the Hospital of Ferrara, Italy, from 1998 to 2001, were considered. After determination of the day of onset, the population was grouped by gender and the most common underlying risk comorbid conditions, e.g., deep vein thrombosis (DVT), neoplasms, cardiomyopathies, traumas/surgical operations, diabetes mellitus, pulmonary diseases, hypertension, cerebrovascular diseases, heart failure, hematologic diseases. For statistical analysis, chi-square test for goodness of fit and partial Fourier series were used. A total of 784 cases (mean age 71 +/- 14 years) were included. Frequency of onset was higher in winter for total population (p = 0.002), men (p = 0.004), DVT (p = 0.001), pulmonary disease (p = 0.008), cardiomyopathies (p = 0.011), and major traumas/surgical operations (p = 0.049). Chronobiologic analysis identified a winter peak for total population (p = 0.008), men (p < 0.001), DVT (p = 0.006), pulmonary diseases (p = 0.017), and hypertension (p = 0.026). This study confirms the winter peak of PE and provides evidence that it is not influenced by the underlying clinical conditions, but probably by endogenous variations.