Background: Probiotic dairy products are increasingly gaining popularity. Although the role of probiotic bacteria in the prevention and treatment of pediatric and antibiotic associated diarrhea is fairly well established, their role in the prevention of adult infectious diarrhea has not been well investigated. Methods: Five hundred forty-one, young male military recruits were enrolled and randomly assigned to receive a yogurt containing Lactobacillus casei (n = 275) or a nonprobiotic yogurt (n = 266). The incidence and duration of diarrhea were documented and stool samples examined for bacteria and parasites. Results: Five hundred and two participants were eligible for final analysis, 254 receiving probiotic yogurt and 248 in the control group. Seventy-one participants (14.14%) experienced diarrhea during the study period. The incidence of diarrhea in the probiotic group and the control group was 12.2% and 16.1%, respectively (P =. 207). The mean duration of diarrhea was 3 ± 1.95 days in the probiotic group and 2.6 ± 1.08 days in the control group (P =. 276). Conclusion: Our study demonstrated a nonsignificant trend for reduction of the incidence of diarrhea among healthy young adults consuming yogurt containing Lactobacillus casei. Further study is needed to evaluate the role of probiotics in adults.