AIMS: To study various aspects of treatment for retinopathy of prematurity (ROP) in a Swedish population of extremely preterm infants born before 27 weeks of gestation. METHODS: A national, prospective and population-based study was performed in Sweden from April 1, 2004 to March 31, 2007. The criteria for treatment of ROP accorded with the recommendations of the Early Treatment for Retinopathy of Prematurity Cooperative Group. RESULTS: Twenty percent of the infants (99/506) were treated for ROP. The likelihood of reaching treatment criteria nearly doubled for each week of reduction in gestational age (GA) at birth. The first treatment was performed at an earlier postmenstrual age in the most immature infants. One third of the infants had more than one session of laser treatment. CONCLUSIONS: A high percentage of these extremely preterm infants required treatment for ROP. The likelihood of reaching treatment criteria increased with a decline in GA at birth. Although only a few infants progressed to ROP Stages 4 and 5, our findings indicate a potential for improvement of the treatment routines, both regarding timing and number of laser spots at the first treatment.