Increasing numbers of people are being readmitted to hospital in England with preventable conditions such as pneumonia and pressure sores, according to Nuffield Trust research.
Better care in hospital and at home afterwards could have made a difference, it said.
Emergency readmissions, within 30 days of discharge, have risen by a fifth over the past seven years.
The government said no-one should endure unnecessary trips to hospital.
It has committed to a long-term funding plan for the NHS to help it manage growing patient demand.
An analysis by the health think tank, the Nuffield Trust, found that a small and growing proportion of emergency readmissions - around 1% - were preventable, affecting 185,000 people last year.
Between 2010-11 and 2016-17, patients readmitted to hospital in an emergency with pneumonia rose from 41,003 to 70,731.
Emergency readmissions for pressure sores almost trebled to 22,448 while patients readmitted with blood clots in a vein (venous thromboembolism) increased by a third to 23,006.
These were conditions that were not diagnosed during their first hospital visit and could have been prevented, the analysis found.
It said the findings should raise questions about the quality of care during a patient's initial hospital stay, how they were discharged from hospital and also the quality of community and social care services.
A growing older population with more complex conditions is one factor in emergency readmissions rising to 1.38 million over the last seven years in England. This is in line with overall admissions.
Emergency readmissions account for 8% of total patient hospital stays, up from 7.5% seven years ago.
Prof John Appleby, director of research at the Nuffield Trust, said: "Unnecessary trips and overnight stays in hospital put a strain on elderly patients and their families.
"That is why it's concerning that our research shows the number of people being readmitted to hospital within 30 days with potentially preventable conditions is greater than it was seven years ago."
Jessica Morris, research author and analyst at the Nuffield Trust, said emergency readmissions to hospital are "potentially a warning sign that a patient's quality of care may have been compromised".
She said local health providers should now focus their attention on the three conditions which had seen the most significant rise in readmissions.
A Department of Health and Social Care spokesman said: "We expect the NHS to work closely with local authorities to ensure people are treated in the most suitable setting and when they are discharged from hospital they have a care plan in place."
Linda Thomas, from the Local Government Association, said adult social care needed to be put on an equal footing to the health service.
"Councils are keen to continue to liaise with their NHS partners locally but need urgent funding to invest in effective prevention work to reduce the need for people to be admitted to hospital in the first place," she said.