Based on wind tunnel experiments in a simulated boundary layer flow, interference effects for a group of two 200 m high reinforced concrete chimneys are investigated, in particular the effect of the natural frequencies of the structure on the response. The wind induced forces are measured by a force balance which acts as a mechanical filter. This distortion has to be removed in the frequency domain. For translating the wind tunnel results to full-scale results, the measured signals have to be weighted additionally by the mechanical admittance of the real structure, taking into account the appropriate frequency scale. After correction and weighting in the frequency domain, the signals are re-transferred to the time domain to sample the extremes of the action effects, i.e., the responses, which now include the quasi-static and resonant contribution. To obtain a sufficiently high statistical stability, 60 independent runs are performed for each interference situation. The analysis of the extremes clearly shows that for interference situations the coefficient of variation of the extremes is high. Therefore, an appropriate fractile of the extremes should be used to define the design wind load. This fractile is obtained assuming a Gumbel distribution for the probability distribution of the extreme action effects. Additionally, an appropriate time scale has to be taken into account. The difference between the proposed fractile and the often applied mean value of the extremes is large, with differences up to 20%. Interference leads to a mixed excitation. Besides resonance due to the turbulence of the oncoming flow, considerable resonant excitation may occur due to wake buffeting. While in the first case a lower value of the natural frequency leads to larger action effects, resonance effects due to wake buffeting become worse for a higher value of the natural frequency. Since in the design calculations the natural frequency can be estimated only within a remaining uncertainty a conservative estimate is often used to determine the wind-induced response. For an isolated chimney excited by wind turbulence, the lower limit of the frequency provides a conservative prediction. For wake buffeting, on the other hand, the upper limit may have more serious effects. Therefore, it is recommended to analyse the structural effects for a sufficiently broad bandwidth of natural frequencies.