Talkers modify the way they speak in the presence of noise. As well as increases in voice level and fundamental frequency (F0), a flattening of spectral tilt is observed. The resulting "Lombard speech" is typically more intelligible than speech produced in quiet, even when level differences are removed. What is the cause of the enhanced intelligibility of Lombard speech? The current study explored the relative contributions to intelligibility of changes in mean F0 and spectral tilt. The roles of F0 and spectral tilt were assessed by measuring the intelligibility gain of non-Lombard speech whose mean F0 and spectrum were manipulated, both independently and in concert, to simulate those of natural Lombard speech. In the presence of speech shaped noise, flattening of spectral tilt contributed greatly to the intelligibility gain of noise-induced speech over speech produced in quiet while an increase in F0 did not have a significant influence. The perceptual effects of spectrum flattening was attributed to its ability of increasing the amount of speech time–frequency plane "glimpsed" in the presence of noise. However, spectral tilt changes alone could not fully account for the intelligibility of Lombard speech. Other changes observed in Lombard speech such as durational modifications may well contribute to intelligibility.