Aim To assess the role of Giardia lamblia and Cryptosporidium parvum as causes of diarrhea outbreaks among children attending child care centers (CCC) during one year of prospective observation. Methods From 11/93 to 11/94, children attending 8 CCCs in the Norfolk, VA, area were monitored daily for occurrence of diarrhea. Stool specimens were collected whenever diarrhea occurred and tested by EIA for giardia and cryptosporidium antigens (LMD Laboratories, Carlsbad, CA) and viral enteropathogens (astrovirus, enteric adenovirus, MX virus, and rotavirus). A diarrhea episode was defined as the occurrence of watery stools at twice a child's daily frequency for at least 2 consecutive days. A diarrhea outbreak was defined as at least 2 diarrhea episodes in the same week in a CCC. An enteropathogen outbreak was defined as a diarrhea outbreak with ≥ 2 children with the same enteropathogen during the outbreak. Results 234 children, 2-31 months of age, were monitored for 4564 child-weeks (mean 20 child-weeks/child). 16 diarrhea outbreaks occurred in 6 of the CCCs; 2 CCCs had no outbreaks. 946 diarrheal stool specimens from 179 children were tested for protozoa. 44 (25%) of the children were infected with protozoa during the study period; 3 children had concurrent infections. 23 (13%) of the children had cryptosporidiosis and 7 of 16 outbreaks included cryptosporidium as a co-pathogen with one or more viral enteropathogens. 27 (15%) of the children had giardiasis. Giardia was the sole cause of 2 diarrhea outbreaks, and a co-pathogen in 9 outbreaks with one or more viral enteropathogens. Giardia more commonly infected children less than 15 months of age than did cryptosporidium (P=.02). Cryptosporidiosis occurred more commonly in Fall/Winter than in Spring/Summer (RR 2.94, 95% CI; 1.19, 7.28), but giardiasis did not have a seasonal predominance. Conclusions During one year of observation, 12% of outbreaks were associated with giardia, and 56% of outbreaks had a protozoal co-pathogen. Giardia lamblia and Cryptosporidium parvum were frequent co-pathogens in CCCs. Giardia infects younger children than cryptosporidium. Cryptosporidium, but not giardia, demonstrated a Fall/Winter predominance in these CCCs.Supported by NIH-AI28544 and 13021, and LMD Laboratories, Carlsbad, CA