Clinical features of traveler's diarrhea (TD) were studied among 126 adult Finnish tourists who developed this illness during or shortly after a visit to Morocco. Enteric pathogens were identified in 76 (60%) of cases, whereas the etiology remained unidentified in 50 cases (40%). Patients with an identified pathogen did not differ from those with TD of unknown etiology in terms of the time of onset of illness or the median frequency of unformed stools in the first 24 hours. In contrast, the median frequency of unformed stools between 24 and 48 hours (i.e., on the second day) was 1.0 among patients with no pathogen and 2.0 among those with enteric pathogens identified (P < .001). A similar difference was evident on the third day (1.0 vs. 2.5). Moreover, a lower proportion of patients with no pathogen identified had watery stools (28% vs. 55%). The durations of diarrhea and concomitant symptoms were significantly shorter and the recovery from TD was significantly quicker among the patients without an identified pathogen. Patients with one or more invasive pathogens had disease that was clearly more severe than that of patients with no pathogen identified; the difference in severity of disease was less marked for patients with invasive vs. noninvasive pathogens. Individuals with diarrhea due to Campylobacter species tended to have the most severe disease, whereas diarrhea caused by enterotoxigenic Escherichia coli seemed milder than that caused by other agents. Unfortunately, the clinician has only a limited opportunity to predict the etiology of TD and thus to assess the need for antimicrobial therapy at the onset of disease.