OBJECTIVE The purpose of this study was to specifically examine the effects of vocal loudness/intensity condition and vowel type on cepstral analysis measurements. METHODS Experimental, mixed design. METHODS Sustained vowel samples of /i/, /ɑ/, /u/, and /æ/ were elicited from 92 healthy male and female subjects between the ages of 18 and 30 years at three different vocal loudness conditions: (1) "Comfortable pitch and loudness," (2) "As softly as possible without whispering," and (3) "As loudly as possible, without screaming/straining the voice or tensing of the neck region." Recordings were made using a calibrated headset microphone and digitized to computer. Vowel samples were analyzed for vocal intensity (decibels), fundamental frequency (F0 in Hertzz), and relative amplitude of the smoothed cepstral peak prominence (CPPs in decibels). RESULTS Significant main effects of loudness condition (with a significant increase in CPPs from quiet to comfortable to loud voice) and gender (males having significantly greater mean CPPs than females) were observed. In addition, results indicated that vowel type had a significant effect on the CPP (greater CPPs for low vowels [/ɑ/ and /æ/] vs high vowels [/i/ and /u/]). CONCLUSIONS The results of this study indicate that both the loudness/intensity of vowel elicitation and vowel type should be monitored closely in both research and clinical situations. Changes in the mode of phonation, as well supraglottal changes that effect vocal tract resonances and vocal intensity, appear to combine to result in substantial differences in the CPPs for different vocal loudness/intensity conditions and vowel types. These results indicate that separate cepstral norms are necessary for vowel type, as well as for gender, when using cepstral analysis as a clinical tool.