This was a multi-center, open-label, two-arm, randomized study. All patients diagnosed with CF and who fulfilled the criteria for early infection with P. aeruginosa initially received tobramycin 300 mg twice a day for 28 days. At the end of the 28-day treatment period, patients who met the inclusion criteria and none of the additional exclusion criteria were randomized in a 1:1 ratio to either receive an additional 28 days of treatment with tobramycin 300 mg twice a day (56-day group) or to stop study medication (28-day group).