Objectives The objective of this study was to determine disease progression, association between neurological signs and magnetic resonance imaging (MRI) findings, and long-term outcome in feline thiamine deficiency associated with defective dry food. Methods The clinical records of 17 cats diagnosed with thiamine deficiency related to a defective dry food were examined and data collected. The thiamine level in the food was analysed by liquid chromatography-tandem mass spectrometry. Results The thiamine level in the food was below the recommendation of the National Research Council. Fifteen cats were fed the food exclusively. Prior to the acute development of neurological signs, most cats displayed non-specific signs such as anorexia, lethargy or vomiting. Vestibular signs of varying severity were observed in 94% of the cats, and all but one of these presented with bilateral dysfunction. Other main neurological signs included altered mentation (76%), blindness (59%) and seizures (59%). Moreover, 80% of the cats with seizures presented with cluster seizures or status epilepticus. MRI abnormalities consistent with findings reported in the previous literature were detected in five cases. MRI was unremarkable in one cat with ongoing severe neurological signs even though thiamine had been administered. Most surviving cats recovered rapidly within 2 weeks of treatment and had either returned to normal or had minimal neurological signs at the 2 month follow-up. One cat recovered slowly over 6 months. Most cats with seizures in the initial stage of the disease remained seizure free at the 24 month follow-up. Conclusions and relevance This study documented the association between feline thiamine deficiency and defective dry food. MRI examination provided valuable information in the diagnosis. However, normal MRI findings do not exclude the diagnosis of feline thiamine deficiency, especially once thiamine has been supplemented. MRI findings also may not always reflect the neurological status or severity. If treated promptly, most cats will recover rapidly with a good outcome. Occasionally, recovery may be slow and take several months.