The Healthcare Commission reviews cases where people complain to an NHS trust about care but are unhappy with the outcome. It looked at 7,827 of these complaints in 2007/08, as well as 1,112 unresolved from the previous year.
The report showed that 11% (984) were about patient treatment, 9% (805) concerned a delay in diagnosing a condition or a wrong diagnosis and 8% (715) were linked with accessing treatment or being forced to wait.
A total of 12% (1,074) were about communication and the information the patient received, while 19% (1,700) were about the way a complaint was handled by the trust.
Some 7% (626) of complaints related to the effectiveness of NHS care, while another 6% (537) concerned the attitude of NHS staff.
'This shows that the NHS still has much room for improvement in how it deals with complaints locally,' the commission said in its report, which focused on the NHS in England.
Almost half (43%) of complaints were about primary care and another 43% were about hospital care, while there were 1,018 complaints about GPs.
However, the commission said these must be seen in the context of about 290 million consultations made by GPs each year in England.
The commission made 12 recommendations to NHS trusts including taking statements and interviewing staff involved in events leading to the complaint.
Related article: 'NHS complaints systems need to be rigorously fair'