OBJECTIVE To investigate the effectiveness of Lactobacillus reuteri DSM 17938 for the treatment of infantile colic in breastfed Canadian infants, compared with placebo. METHODS A randomized, double-blind, placebo-controlled trial was conducted involving 52 infants with colic, according to modified Wessel criteria, who were assigned at random to receive L reuteri DSM 17938 (10(8) colony-forming units) (n = 24) or placebo (n = 28) for 21 days. Daily crying and fussing times were recorded in a structured diary, and maternal questionnaires were completed to monitor changes in infant colic symptoms and adverse events. RESULTS Total average crying and fussing times throughout the study (from baseline to day 21) were significantly shorter among infants with colic in the probiotic group compared with infants in the placebo group (1719 ± 750 minutes [29 ± 13 hours] vs 2195 ± 764 minutes [37 ± 13 hours]; P = .028) (relative risk, 0.78; 95% CI, 0.58-0.98). Infants given L reuteri DSM 17938 showed a significant reduction in daily crying and fussing times at the end of treatment period compared with those receiving placebo (median, 60 minutes/day [IQR, 64 minutes/day] vs 102 minutes/day [IQR, 87 minutes/day]; P = .045). On day 21, a significantly higher proportion of infants in the L reuteri DSM 17938 group responded to treatment with a ≥50% crying time reduction compared with infants given placebo (17 vs 6, P = .035; relative risk, 3.3; 95% CI, 1.55-7.03). CONCLUSIONS Administration of L reuteri DSM 17938 significantly improved colic symptoms by reducing crying and fussing times in breastfed Canadian infants with colic.