AIM: To investigate recruitment, retention, and estimates for effects of formula supplementation with Lactobacillus rhamnosus GG(LGG) on inflammatory biomarkers and fecal microbial community in infants with colic. METHODS: A prospective, double-blind, placebocontrolled trial was conducted in otherwise healthy infants with colic. We screened 74 infants and randomized and analyzed results in 20 infants [9 receiving LGG(LGG+) and 11 not receiving LGG(LGG-)]. LGG was incorporated in the formula(Nutramigen~®)(minimum of 3 × 107 CFU/d) in the LGG+ group. Fecal microbiota and inflammatory biomarkers, including fecal calprotectin(FC), plasma cytokines, circulating regulatory T cells(Tregs), and crying + fussing time were analyzed to determine optimal time points and effect sizes for a larger trial. RESULTS: Recruitment in this population was slow, with about 66% of eligible infants willing to enroll; subject retention was better(75%). These rates were influenced by parents’ reluctance to volunteer their infant for a clinical trial and by their tendency to change formulas. The maximal difference of crying + fussing time was observed at day 14, comparing the 2 groups, with a mean difference of-91(95%CI:-76, 259) min(P = NS). FC showed no significant difference, but the optimal time to determine a potential effect was at day 90 [with a mean difference of 121(95%CI:-48, 291) μg/g stool], observing a lower level of FC in the LGG+ group. The fecal microbial communities were chaotic, as determined by Shannon’s diversity index and not apparently influenced by the probiotic. No significant change was observed in plasma inflammatory cytokines or Tregs, comparing LGG+ to LGG- groups. CONCLUSION: Designing future colic trials involving a probiotic-supplemented formula for infants in the United States will require consideration for difficult enrollment. Infants with colic have major variations in feal microbiota and calprotectin, both of which improve with time, with optimal time points for measurement at days 14 and 90 after treatment. AIM: To investigate recruitment, retention, and estimates for effects of formula supplementation with Lactobacillus rhamnosus GG (LGG) on inflammatory biomarkers and fecal microbial communities in infants with colic. METHODS: A prospective, double-blind, placebocontrolled trial was conducted in otherwise healthy infants with colic. We screened 74 infants and randomized and analyzed results in 20 infants [9 receiving LGG (LGG +) and 11 not receiving LGG (LGG-)]. LGG was incorporated in the formula (Nutramigen ~ 107 CFU / d) in the LGG + group. Fecal microbiota and inflammatory biomarkers, including fecal calprotectin (FC), plasma cytokines, circulating regulatory T cells (Tregs), and crying + fussing time were analyzed to determine optimal time points and effect sizes for a larger trial. RESULTS: Recruitment in this population was slow, with about 66% of eligible infants willing to enroll; subject retention was better (75%). These rates were influenced by parents’ reluctance to vol unteer their infant for a clinical trial and by their tendency to change formulas. The maximal difference of crying + fussing time was observed at day 14, comparing the 2 groups, with a mean difference of-91 (95% CI: -76, 259 ) min (P = NS). FC showed no significant difference, but the optimal time was determined to have a potential effect was at day 90 [95% CI: -48, 291 μg / g stool] The fecal microbial communities were chaotic, as determined by Shannon’s diversity index and not apparently influenced by the probiotic. No significant change was observed in plasma inflammatory cytokines or Tregs, comparing LGG + to LGG-groups CONCLUSION: Designing future future trials involving a probiotic-supplemented formula for infants in the United States will require consideration for difficult enrollments. Infants with colic have major variations in feal microbiota and calprotectin, both of which improve with time, with optimal time ppoints for measurement at day 14 and 90 after treatment.