Immunohistochemistry for pimonidazole adducts serves to define hypoxia within tissues. For this purpose, pimonidazole is delivered in vivo, binds to thiol groups at oxygen tensions below 10 mmHg, and is visualized with help of commercially available anti-pimonidazole antibodies. Renal parenchymal oxygen distribution is highly variable under normal conditions and during acute renal failure and chronic renal disorders. Pimonidazole immunostaining clearly helps in delineating hypoxic regions within the kidneys, but technical pitfalls should be taken into account. In particular, tissue fixation by in vivo perfusion is strongly recommended in order to eliminate artificial staining, because immersion fixation per se can promote a hypoxic environment within kidney tissue.