The 75-year-old former Uttar Pradesh chief minister came to Medicity Medanta Hospital on Friday after feeling unwell. Doctors have advised him three days rest.
A hospital source told IANS that Yadav, a former defence minister, had been admitted in a private room.
"The patient is undergoing several tests including ultrasound and CT scan," the source said.
Yadav was admitted to the same hospital in March following an infection in his abdomen.
But in Lucknow, the Samajwadi Party denied that Yadav was unwell.
A close aide told IANS that Yadav had gone to Gurgaon for a "follow-up check-up". He said: "Netaji is hale and hearty and in the pink of health."