Objectives:Cholecystokinin (CCK) plays an important role in postprandial gallbladder contraction and may also have pacifying behavioral effects, such as inducing satiety and calming in infants. The authors hypothesized that colicky infants have impaired CCK secretion, which contributes to their gallbladder hypocontractility (reported earlier) and excessive crying.Methods:Cholecystokinin levels of 40 colicky and 37 control infants were evaluated at a mean of age 5 weeks. Plasma CCK levels were drawn before, immediately after, and 1 hour after a regular milk feeding and analyzed using radioimmunoassay (RIA). The contraction of the gallbladder was calculated using ultrasound measurements before and 1 hour after the feeding. The two‐tailttest for independent samples was used to test continuous data.Results:The preprandial and 1‐hour postprandial plasma levels of CCK were lower in the colicky infants than in the control subjects (P< 0.05). The immediate postprandial CCK levels were also lower in the colicky infants, although this difference did not reach a statistical significance. The proportion of gallbladder contractions did not differ between the study groups.Conclusions:The authors conclude that low CCK levels in colicky infants may predispose them to excessive crying in the absence of the calming effect of CCK.