This is a case study of a 6-week-old infant referred for colic whose mother was using fluoxetine hydrochloride and breast feeding the infant. The mother switched to a commercial formula and reported a dramatic decrease in the infant's crying. We asked the mother to feed the infant breast milk from a bottle and she agreed. Throughout the study the mother kept a daily diary of her infant's crying, sleeping, stooling patterns, and feeding problems. Analysis of the mother's breast milk showed concentrations of 69 ng/mL for fluoxetine and 90 ng/mL for norfluoxetine. Infant blood serum/plasma level was analyzed for fluoxetine hydrochloride following return to breast milk. The concentrations were 340 ng/mL for fluoxetine and 208 ng/mL for norfluoxetine. The diary records showed increased crying, decreased sleep, increased vomiting, and watery stools when fluoxetine hydrochloride was transmitted through breast feeding or breast milk in bottle. These symptoms were reduced when the infant was formula fed. We suggest a possible relationship between colic and associated symptoms and fluoxetine hydrochloride in maternal breast milk. J. Am. Acad. Child Adolesc. Psychiatry, 1993, 32, 6:1253–1255.