The epidemiology of infectious diarrhea influences the microbiological investigation of the disease, and is best elucidated through prospective studies. We undertook such a study in a Nordic country. Patients of all age groups who had acute diarrhea were recruited prospectively from general practice clinics in Iceland. They completed a questionnaire and provided stool samples for the detection of pathogenic viruses, bacteria, and parasites. Of the 464 recruited patients, 211 (45%) had 224 infections. The most common pathogens were calici- and rotaviruses (23% and 18% of 224 infections, respectively), Campylobacter jejuni (17%), Cryptosporidium species (12%), and Salmonella serotypes (10%). Other agents found were Giardia lamblia, astro- and adenoviruses, and Yersinia enterocolitica. Viral pathogens were associated with vomiting, illness for ≤7 days, and younger age groups, while bacterial pathogens were associated with fever, rectal pain, and local or foreign travel, and parasitic pathogens with illness for >7 days. Detection rates for viral infections in the elderly and for Cryptosporidium species were higher than expected. Our study indicates the existence of regional differences in pathogen prevalence that should be taken into account when implementing guidelines for stool testing in patients with acute diarrheal disease.