diff --git a/base/bitarray.jl b/base/bitarray.jl
index 2bbb138c1d7e5..5e2bc4bcfde10 100644
--- a/base/bitarray.jl
+++ b/base/bitarray.jl
@@ -15,6 +15,11 @@ over `Array{Bool, N}` and allowing some operations to work on 64 values at once.
 By default, Julia returns `BitArrays` from [broadcasting](@ref Broadcasting) operations
 that generate boolean elements (including dotted-comparisons like `.==`) as well as from
 the functions [`trues`](@ref) and [`falses`](@ref).
+
+!!! note
+    Due to its packed storage format, concurrent access to the elements of a `BitArray`
+    where at least one of them is a write is not thread safe.
+
 """
 mutable struct BitArray{N} <: AbstractArray{Bool, N}
     chunks::Vector{UInt64}