West Midlands Ambulance Service tweeted pictures of the hand-written message, after responding to a call in Birmingham on Friday.
The note read: "You may be saving lives, but don't park your van in a stupid place and block my drive."
The ambulance service expressed disbelief and said they had been treating a man vomiting blood in the Small Heath area of the city, at the time.
In a statement, paramedics said: "Sometimes we just don't know what to say.
"This was the note left on an ambulance today.
"At the time, the crew were helping a man who was extremely unwell after vomiting blood.
"They took him on blue lights to hospital where he was in a critical condition."
Following a wave of support on social media, the ambulance service thanked people for their backing.
The paramedics said: "We still can't believe it, and it appears you can't either.
"Leaves us feeling really rather #sad - thanks again for all your support."