Purpose To analyse the immediate effect of a vocal warm-up program on children that sang in a choir by means of spectrographic analysis. Methods Experimental study consisting of 14 girls from an amateur children’s choir, aged 9 to 12 years. Acoustic spectrographic analyses were performed before and after a specific vocal warm-up program, consisting of a hierarchical sequence of body exercises, respiratory and articulation training, vocal exercises, ascending and descending musical scales and facilitating sounds. The speech material for spectrographic analysis was the sustained /Ɛ:/ vowel, pre and post warm-up program. The selected acoustic parameters were instability of trace, noise at low and high frequencies, frequency bifurcation, harmonics series, presence of a hard vocal attack, pitch and voice breaks, and mean fundamental frequency. Results No significant differences were found regarding the effect of the vocal warm-up when pre and post data were compared. However, there was a negative correlation between the level of harmonics and noise in the high frequencies post warm-up. Conclusion The vocal warm-up program implemented for children that sang in choirs produced a richer acoustic spectrum that may indicate enhanced glottic closure or vocal tract adjustment.