Retinopathy of prematurity (ROP) is found in 10% of patients in neonatological units. Most often, there is spontaneous restitution. Rarely, severe degrees develop (retrolental fibroplasia); occasionally, infants become blind even nowadays. The etiological role of oxygen is undisputed. Additional factors, however, are also found with significant frequency in the history of infants with ROP. These are elevated pCO2-values, an acidotic pH, and blood transfusions. These three parameters lead to increased availability of oxygen for the tissue. Data from animal experiments confirm this effect. The statistical significance of these factors is clear, whereas their clinical significance has not yet been proved. Probably, the most effective prophylactic measure for preventing severe degrees of ROP is the repeated examination of newborns presenting defined risks by especially trained ophthalmologists.