Inclusion criteria  Diagnosis of GPP confirmed based on the Japanese Dermatological Association (JDA) guidelines for the management and treatment of GPP. Patient is experiencing a flare, defined as new or worsening of widespread eruption of sterile macroscopically visible pustules, with or without systemic inflammation, as assessed by the investigator. Male or female patients, aged 20 to 75 years at time of enrollment. Women of childbearing potential (WOCBP) must be willing and able to use a highly effective method of birth control per International Council for Harmonization (ICH) M3 (R2) that result in a low failure rate of less than 1% per year when used consistently and correctly. A list of contraception methods meeting these criteria is provided in the patient information. Signed and dated written informed consent in accordance with International Council for Harmonization-Good Clinical Practice (ICH-GCP) and local legislation prior to admission to the trial. No satisfactory authorised alternative therapy exists, as assessed by the investigator.  Exclusion criteria  Women who are pregnant, nursing, or who plan to become pregnant while in the trial.  -- Women who stop nursing before study drug administration do not need to be excluded from participating; they should refrain from breastfeeding for 16 weeks after the last spesolimab infusion.  Severe, progressive, or uncontrolled hepatic disease, defined as >3-fold Upper Level of Normal (ULN) elevation in Aspartate Transaminase (AST) or Alanine Aminotransferase (ALT) or alkaline phosphatase, or >2-fold ULN elevation in total bilirubin. Active systemic infections (fungal and bacterial disease) during the last 2 weeks prior to drug administration, as assessed by the investigator. Increased risk of infectious complications (e.g. recent pyogenic infection, any congenital or acquired immunodeficiency (e.g. Human Immunodeficiency Virus (HIV)), past organ or stem cell transplantation), as assessed by the investigator.  Relevant chronic or acute infections, including active tuberculosis (TB), HIV infection or viral hepatitis at the time of drug administration.  Patients should be evaluated for TB infection prior to initiating treatment with spesolimab. Anti-TB therapy should be considered, in accordance with local guidelines, prior to initiating spesolimab in patients with latent TB or a history of TB. History of allergy / hypersensitivity to systemically administered spesolimab or its excipients. Any documented active or suspected malignancy or history of malignancy within 5 years prior to screening, except appropriately treated basal or squamous cell carcinoma of the skin or in situ carcinoma of uterine cervix. Immediate life-threatening flare of GPP requiring intensive care treatment according to the investigator's judgement. Life-threatening complications include cardiovascular / cytokine driven shock, pulmonary distress syndrome, or renal failure.  Further exclusion criteria apply.