  We study the electromagnetic on-shell form factor of quarks in massless
perturbative QCD. We derive the complete pole part in dimensional
regularization at three loops, and extend the resummation of the form factor to
the next-to-next-to-leading contributions. These results are employed to
evaluate the infrared finite absolute ratio of the time-like and space-like
form factors up to the fourth order in the strong coupling constant. Besides
for the pole structure of higher-loop QCD amplitudes, our new contributions to
the form factor are also relevant for the high-energy limit of massive gauge
theories like QED. The highest-transcendentality component of our results
confirms a result recently obtained in N=4 Super-Yang-Mills theory.
