Chronic Renal Failure

• Chronic renal failure is a serious kidney disease. A symptom of chronic renal failure in cats is when they vomit clear or white foam.
  
  

Feline Parvovirus

• Feline parvovirus is also known as FPV or Feline Infectious Enteritis, FIE. Symptoms include foamy vomit, fever and, in more serious cases, sudden death. A cat with feline parvovirus will try to eat or drink, but will be unsuccessful. A stool sample will be needed for your veterinarian to make an accurate diagnosis.
  
  

Plants

• Cats are constantly looking for new things to explore, including plants. If a cat vomits a substance of a foamy, green consistency, this could be an indicator that the cat may have eaten a toxic plant.
  
  

Empty Stomach

• Cats may vomit a clear, foamy liquid if they are nauseous and have an empty stomach.
  
  

Normal

• In some cats, vomiting a foamy substance is a normal reaction to stress. According to Veterinary Associates, "Most healthy cats will, on occasion, vomit whole or partially digested food, hairballs, or foamy, clear liquid."