More than a million people suffering from asthma are missing out on key yearly checks.
New analysis by charity Asthma UK found that 31% of asthma patients did not receive an "essential" annual review to check whether they are on the right medicine.
The charity said that its review of GP data for 2012/13 showed that there were 3,359,612 people in England who should have received an asthma review but 1,025,539 patients missed out.
NHS guidance suggests that everyone with asthma should get an annual review, an asthma action plan and their inhaler technique checked.
"With the worrying scale of prescribing errors identified by the National Review of Asthma Deaths, it's vital that doctors and nurses do everything they can to follow up with patients to review their medicines, especially as asthma can vary hugely over the year.
"There is also an unacceptably large variation in the numbers of people attending annual reviews, which ranges from only 52% to 79% across the UK," a spokeswoman said.