Chicken salad prepared by a party hostess the day prior to a social event was epidemiologically associated, by case-control study, with an outbreak of gastrointestinal illness caused by Cryptosporidium parvum. The ingredients for the salad were: cooked chopped chicken; pasta; chopped hard-boiled eggs; chopped celery; chopped grapes; seasoned mayonnaise dressing. The hostess prepared the salad while she ran her home-based, child care program. She denied recent diarrheal illness and refused to submit a stool specimen for analysis. Food was not available for testing. Two ill persons were confirmed to have C. parvum in their stool. C. parvum is most often transmitted through the consumption of contaminated water. It can be passed directly from animal-to-person or person-to-person. Outbreaks of C. parvum and asymptomatic carriage of C. parvum in child care programs had been documented previously.