We conducted a point prevalence survey for enteric protozoa in 205 institutionalized orphans 1-61 months of age in Bangkok, Thailand. Cryptosporidium was identified in 17 children (8%), Giardia lamblia in 42 (20%), and 3 children (1%) had both parasites. At the time of diagnosis, diarrheal symptoms were present in a minority of subjects: 36% of children with Cryptosporidium alone, 10% with G. lamblia alone, and in 20% of those with neither parasite. Although chronic nutritional status (height/age) was similar in all groups, acute nutritional status (weight/height) was lower only in children with Cryptosporidium (Z score = -1.39 +/- 0.13) compared with children with G. lamblia (mean Z score +/- SEM = -0.56 +/- 0.26) or neither parasite (Z score = -0.78 +/- 0.13; P = 0.05). Detectable levels of Cryptosporidium-specific IgG antibodies by ELISA were identified in 15 of 16 Thai children with Cryptosporidium and in 17 of 19 Thai children without Cryptosporidium (mean OD +/- SEM = 1.27 +/- 0.18 vs. 1.06 +/- 0.13, respectively), but in only 1 of 18 sera from toddlers in day-care centers in Denver, CO (OD = 0.128 +/- 0.03). Although neither infection with Cryptosporidium nor G. lamblia was consistently associated with acute diarrheal symptoms, Cryptosporidium was more often associated with depressed acute nutritional status than G. lamblia. The high prevalence of specific antibodies to Cryptosporidium in Thai orphans suggests an association between high rates of exposure with asymptomatic excretion of the parasites.