We establish sharp (or 'refined') comparison theorems for the Klein-Gordon equation. We show that the condition Va ≤ V b , which leads to Ea ≤ E b , can be replaced by the weaker assumption Ua ≤ U b which still implies the spectral ordering Ea ≤ E b . In the simplest case, for d = 1,Vi(t)dt, i = a or b, and for d > 1, Ui(r) = r 0 Vi(t)t d−1 dt, i = a or b. We also consider sharp comparison theorems in the presence of a scalar potential S (a 'variable mass') in addition to the vector term V (the time component of a 4-vector). The theorems are illustrated by a variety of explicit detailed examples.