One-third of 5,792 fecal specimens from 2,896 patients in 48 states and the District of Columbia tested positive for intestinal parasites during the year 2000. Multiple infections with 2-4 parasitic species constituted 10% of 916 infected cases. Blastocystis hominis infected 662 patients (23% or 72% of the 916 cases). Its prevalence appears to be increasing in recent years. Eighteen other species of intestinal parasites were identified. Cryptosporidium parvum and Entamoeba histolytica/E. dispar ranked second and third in prevalence, respectively. Prevalence of infection was lowest (22-27%) in winter, gradually increased during the spring, reached peaks of 36-43% between July and October, and gradually decreased to 32% in December. A new superior method of parasite detection using the Proto-fix-CONSED system for fixing, transport, and processing of fecal specimens is described. In single infections, pathogenic protozoa caused asymptomatic subclinical infections in 0-31 % of the cases and non-pathogenic protozoa unexpectedly caused symptoms in 73-100% of the cases. The relationship between Charcot-Leyden crystals and infection with four species of intestinal parasites is examined and the list of provoking parasitic causes is expanded.