This paper presents the results of an analysis of flow-induced oscillations of a square section cylinder under interference conditions using a data-mining tool called 'decision tree'. The interference effects were studied at some specific relative positions identified. Experiments have been carried out for various relative dimensions or size ratios (b/B) of the test cylinder and the interfering cylinder with values of 0.5, 1.0, 1.5 and 2.0. It has been found that the parameters reduced velocity (U/fB), relative position (L/B, T/B) and size ratio (b/B) influence the flow-induced oscillation of the cylinder quite significantly. In practical situations, very often, critical combinations of these parameters leading to objectionable vibratory amplitudes may occur and, hence, they need to be identified and eliminated. It is here the application of 'decision tree' found to be significantly helpful. Hence, in this paper, emphasis is laid on the effectiveness of 'decision tree' in analysing the flow-induced vibration data and consequently arriving at the safest as well as the critical conditions. The results show that, for safer design conditions, reduced velocity should be lower than a threshold value. It has been also found that relative position is playing only a lesser significant role when compared to reduced velocity and size ratio. The results further show that critical conditions are very likely to occur at high reduced velocities, for size ratios greater than one. © 2007 Elsevier Ltd. All rights reserved.