The marine growth of approximately 20 000 tagged and recaptured sea trout (Salmo trutta trutta) smolts was examined following Finnish releases into the Baltic Sea from 1980 to 2010. All these trout smolts were hatchery reared. Due to the low catch length and intensive fishing, a high proportion of the trout were captured in their first and second sea year, i.e. before spawning. Sea trout grew better in the southern Finnish sea area (59 degrees 30'-60 degrees 30'N) than in its northern parts (60 degrees 30'-66 degrees N). The marine growth rate increased in both sea areas from 1980 to 2010, but relatively more in the northern than the southern one, especially among the northern two-sea-winter-old trout. Better annual marine growth was associated with an increase in the sea surface temperature in April, or a high abundance of Baltic herring (Clupea harengus membras). The condition factor of trout was higher in the southern than the northern sea area and was positively linked to herring abundance.