The relationship between oral temperature and other parameters of illness was examined in 139 adult volunteers infected experimentally with Shigella sonnei. In subjects developing clinical disease, peak temperature correlated positively with total number of signs and symptoms other than fever (rxy = .71, P < .001), stool volume (rxy = .41, P < .001) and number of stools produced during the illness (rxy = .46, P < .001). Peak temperature correlated negatively with incubation period (rxy = -.34, P = .007) but exhibited no apparent correlation with duration of illness. The average oral temperature during illness correlated positively with number of other signs and symptoms of infection but not with stool volume or stool number. These results suggest that in clinical investigations involving S. sonnei, and perhaps with other pathogenic microorganisms, oral temperature is a useful quantitative marker for estimating disease severity.