The most severe form of rapidly progressing retinopathy of prematurity (ROP) is termed aggressive posterior ROP (APROP). APROP frequently causes severe visual impairment in affected preterm infants despite timely and appropriate laser treatment. We investigated the postnatal characteristics associated with APROP development in a national Swedish cohort. This retrospective, 1: 1 matched case-control study included all infants that developed APROP in zone 1 (n= 9) between 2008 and 2012. Control infants, matched for gestational age and birth weight, developed ROP no worse than stage 2 (n= 9). We retrieved data from medical records on infant birth characteristics, postnatal morbidities, and blood analyses from birth to the first ROP treatment. Infectious episodes included sepsis, C-reactive protein≥ 10 mg/l, and other clinical signs of infection that required antibiotic treatment. A platelet count< 100× 10 9/l was considered to be thrombocytopenia. All APROP cases postnatally developed at least two infectious episodes, one in the first month and one around the time of ROP diagnosis. All APROP cases exhibited thrombocytopenia in the first month, and 6/9 exhibited thrombocytopenia around the time of ROP diagnosis. Compared to the controls, APROP cases more frequently developed necrotizing enterocolitis (8/9 vs. 1/9; p< 0.01) and sepsis (9/9 vs. 3/9; p< 0.01), and they had significantly lower median platelet counts (90× 10 9/l, range 4-459, vs. 158× 10 9/l, range 20-500; p< 0.001). Multiple infectious episodes and thrombocytopenia, particularly around the time of ROP diagnosis, were associated with APROP development.