Another thread could 1) write to the same key. 2) cause a table expansion (or a shrink) which rehashes the whole table (in most implementations). Either case will cause a difficult to diagnose bug. Use a lock or communicate with channels instead. There may be other subtle things which can get corrupted under concurrent use. The map type in Go has never been implemented in a thread safe fashion so if you try and use it as such you will be in for a bad time.