The German Working Group on Indoor Guidelines of the Indoor Air Hygiene Committee and of the Supreme State Health Authorities is issuing indoor air guide values to protect public health. No reliable human studies are available for health evaluation of methyl isobutyl ketone (MIBK) in indoor air. In a well documented chronic inhalation animal study with rats assessed as reliable, nephrotoxic effects were observed, which led to an increased incidence and severity of the chronic progressive nephropathy, especially in females. Using a benchmark approach the Working Group assessed a BMDL10 of 57 mg MIBK/m³ for continuous exposure for the endpoint nephrotoxicity. By applying an interspecies factor of 2.5, a factor of 10 for interindividual variability, and a factor of 2 to account for the higher respiratory rate of children compared to adults, a health hazard guide value (RW II) of 1 mg MIBK/m³ indoor air is obtained. A precautionary guide value of 0.1 mg MIBK/m³ indoor air is recommended.