The aim of the study was to define the respiratory morbidity caused by lymphocytic interstitial pneumonitis (LIP) in children with vertically acquired HIV infection. A retrospective case note review was performed on 95 children attending three London hospitals. Clinical and radiological evidence of LIP, acute lower respiratory tract infections, and chronic lung disease was obtained using a structured protocol. A diagnosis of LIP had been made in 33%, and an acute admission due to acute lower respiratory tract infection had occurred in 42% of all children (despite 99% taking regular cotrimoxazole prophylaxis). Admission rates because of acute lower respiratory tract infection were significantly higher in the LIP group (0.38 admissions/child year) than in the non-LIP group (0.17 admissions/child year) (p = 0.0002). Encapsulated bacteria (Streptococcus pneumoniae, Haemophilus influenzae) were most frequently isolated. Improved methods of prevention of acute lower respiratory tract infection may help to reduce the severe respiratory morbidity seen in children with LIP and HIV infection.