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My dog has a big, fluid-filled, painless armpit lump. What is it?

Updated On September 23rd, 2025

Pet's info: Dog | Mixed Breed Large (61lb +) | Female | spayed | 80 lbs

My dog has a very large, softball sized lump in her armpit. It seems to be fluid filled. It is not giving her pain. What is this?

2 Answers

Most Helpful Answer

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Answered By Dr. Melanie, BVSc MS

Veterinarian

Published on November 29th, 2016

This lump could be a cyst or an abscess. I recommend taking Roxy into the vet tomorrow for an exam. The vet can do a physical exam, and can do a fine needle aspirate on the lump. To do this, the vet sticks a small needle into the lump to get a sample of it's cells, and examines the cells on a slide under a microscope. It's usually painless to the animal, and relatively quick to perform. If it is a cyst, it will need to be drained, and it may need to be removed in order to keep it from returning. An abscess will require it to be drained and flushed sterilely, and she would need antibiotics to treat the infection.

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5Pet Parents found this answer helpful

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    Answered By Ricardo Fernandez, DVM

    Veterinarian, Veterinary Oncology Resident

    Published on September 26th, 2017

    Thanks for using Petco Pet Education Center, formerly Petcoach! Given that this has only been present for one day and has increased in size rapidly and has some discharge associated with it, this may be an abscess. Otherwise it could be a benign or malignant growth. I recommend you have Ronnie evaluated by your veterinarian for a test called fine needle aspirate which is an easy, quick and pain-free procedure which in the majority of cases will help reach a diagnosis to identify what the best course of action is based on the findings. In some cases, a biopsy is needed for a diagnosis. If it is an abscess, drainage and antibiotic therapy will be recommended. If it turns out to be a tumor, then treatment will be recommended according to the tumor type and other findings. Good luck! Keeping my fingers crossed that it is something benign.

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