A microcomputer-based approach to the quantification of facial expression was used to measure and compare the smiling behavior of a group of Parkinson's disease sufferers, a group of patients with major depression, and a control group of comparable age. Subjects were asked to view a series of amusing slides and their expressions were recorded. The most animated smile for each subject was chosen for analysis and scores on 12 computer-generated measures were obtained using the Facial Expression Measurement program. These measures are end-lip measure, mouth width measure, mouth-opening measure, mid-top measure, mid-lower lip measure, top lip-thickness measure, lower lip-thickness measure, eye-opening measure, top eyelid/iris intersect measure, lower eyelid/iris intersect measure, inner eyebrow measure, and mid-eyebrow measure. The depressed group differed significantly from the other groups, with higher scores on end-lip measure, mid-top lip measure, and mid-eyebrow measure. All subjects completed the Levine-Pilowsky Depression Questionnaire. The depressed patients obtained higher depression scores than the parkinsonian group, who in turn had significantly higher depression scores than the control group. The depression score was correlated with end-lip measure, mouth width measure, mid-top lip measure, eye-opening measure, and mid-eyebrow measure in the population as a whole. A significant negative correlation emerged between the depression score and mid-eyebrow measure in the depressed group. Both the depressed group and the parkinsonian group were found to smile significantly less often during the slide session when compared with the control group. These results are discussed in the light of earlier findings.