In this paper we study the correlation of node failures in time and space. Our study is based on measurements of a production high performance computer over an 8-month time period. We draw possible types of correlations between node failures and show that, in many cases, there are direct correlations between observed node failures. The significance of such a study is twofold: achieving a clearer understanding of correlations between node failures and enabling failure detection as early as possible. The results of this study are aimed at helping the system administrators minimize (or even prevent) the destructive effects of correlated node failures.