Different studies have shown that some clinical events, particularly cardiovascular and thrombotic events, show a regularity in its appearance. The aim of our study was to analyse the possible existence of seasonal periodicity in the incidence of the vascular access thrombosis in patients on chronic haemodialysis. Prospectively, we collected information of 164 patients with 250 episodes of vascular access thrombosis referred to our hospital from january 1995 to december 1999. An ANOVA test for comparison of the means, and a time series analysis were performed. During the five year study the consecutive number of thrombosis were 43, 57, 55, 59 and 36. When the different seasons were analysed, the cumulative number of events in summer during the study period were 91, a significant increase compared to spring, autumn, and winter (54, 54, and 51, respectively; p<0.001). Time series analysis confirmed that thrombolic events during summer showed an increased incidence over the mean (p<0.001), and it occurred every year. The same results were obtained when the PTFE grafts were analyzed separetely (july RR 2.62, p=0.002; august, RR 2.37, p=0.04), but not with the arteriovenous fistulae. In conclusion, this study showed a seasonal periodicity of vascular access thrombosis, with a PTFE graft. Although the causes were unknown, these data alert us on the convenience of an increased attention to the vascular access during the summer months in order to prevent its thrombosis.