Recorded crackling lung sounds of 11 patients with pneumonia were studied with phonopneumography, FFT spectrography and time-expanded waveform display. The sounds were recorded on average six days after the onset of pneumonia and the recording was repeated two to four days later. In the first recording the crackles were coarse and midinspiratory. The patients with unilateral pneumonia had a significant difference in the upper frequency limit of inspiratory sound of the FFT spectrum between the healthy and diseased lung (p less than 0.01). In the second recording, the beginning of crackling had shifted later (p less than 0.01) and the end point of crackling also became later (p less than 0.05). The largest deflection width of the individual crackles became shorter (p less than 0.05). The results indicate that the pneumonic crackles vary markedly during the clinical course of pneumonia. The duration of the individual crackles became shorter and the timing of the crackles shifted toward the end of inspiration.