Lower respiratory tract infections (LRTIs) are a driving factor for lung function decline in children with cystic fibrosis (CF). The respiratory microbiota is closely related to the pathogenesis of LRTIs in healthy infants, data in CF infants is scarce. We compared nasal microbiota development between CF and healthy infants and assessed associations between early-life nasal microbiota, LRTIs and antibiotic treatment in CF infants. We analyzed the microbiota after amplification of the V3-V4 region of the 16S rRNA gene from 1511 biweekly nasal samples from 50 CF and 30 healthy infants. Microbiota diversity differed between CF infants and healthy controls following LRTIs and/or antibiotic treatment. CF infants with a lower α-diversity presented with a subsequent higher number of LRTIs. Early nasal microbiota alterations may contribute to an increased susceptibility to LRTIs in CF infants and may further increase after LRTIs and antibiotic treatment. This emphasizes the possible preventive potential of targeting the nasal microbiota in CF-related LRTI management.