Exclusive breastfeeding (EBF) is defined as feeding babies with only breast milk and not any other foods or liquids, except for medications or vitamin and mineral supplements before the age of 6 months. In a setting with intensive support for breastfeeding and child health practices, we documented level and reasons for EBF interruption, evaluated factors associated with interruption of EBF and compared linear growth outcomes in the EBF interruption categories. We conducted a retrospective longitudinal cohort study of a random sample of 366 out of 600 infants enrolled in a trial in Homa Bay County, western Kenya (PROSYNK). The trial assessed whether dietary supplementation of newborns with pro/synbiotics improved gut health and reduced systemic inflammation. Socio- demographic and anthropometric data were obtained from cleaned trial data. When and why interruption of EBF occurred were abstracted from participant‟s home visit records. Chi-square and Fisher‟s exact tests used to compare categorical variables. Bivariate and multivariate analysis used to investigate further associations. Changes of SD for LAZ over time in the EBF interrupted categories were graphically illustrated. EBF was interrupted in 165/366 (45.1%) infants. Perception that breast milk was not enough was the major reason for interruption (83; 50.3%). Reasons for EBF interruption differed according to timing, early interruption was mostly due to school resumption (48%, 23/48) while late interruption was mostly due to perception that breastmilk was not enough (53%, 62/117) p=0.002. Interrupted EBF was associated with mothers who were younger (p=0.01), single (p<0.01) and with fewer children (p<0.04). Linear growth of infants with early interruption fluctuated greatly over time. EBF was uncommon despite comprehensive support. Support for breastfeeding to younger mothers, especially those who are unmarried and still attending school, needs to be prioritized. This study holds significance, highlights specific groups of lactating mothers who need special focus for breastfeeding programing.