Parasitic diseases are very important in Mexico because of their economic impact and adverse effects on normal growth in children. Cryptosporidiosis has been associated with acute diarrhea in immune competent and incompetent human hosts, fecal contamination of drinking water sources, and handling of animals. Due to the lack of reports on cryptosporidiosis in Mexico, we conducted a parasitologic study in children with diarrhea and other clinical symptoms. The main objectives were 1) to determine the prevalence of cryptosporidiosis in children less than one year of age in Mexico City, and 2) to correlate Cryptosporidium infection with gastrointestinal symptoms. Two hundred fecal samples from children seen at the Gabriel Mancera Familiar Medicine Unit of the Instituto Mexicano del Seguro Social were studied. Children were divided into two groups. Group A was composed of sick children with 6-8 watery diarrheic episodes every 24 hours attended at the emergency service. Group B was composed of healthy babies getting routine check ups. Only children in group A were found to be infected with intestinal protozoa (50% with Giardia lamblia, 41% with Cryptosporidium spp., and 4% with Entamoeba histolytica). The results suggested a high incidence of Cyrptosporidium infections in children in Mexico City, which make these observations useful for future studies.