Australian fertility specialist David Greening recruited 118 men whose sperm had a higher-than-normal level of DNA damage.
Before the test, on average 34 percent of the group's sperm was rated as damaged, meaning that it was classified as "poor" in quality. Among individuals, this ranged from 15 percent to 98 percent.
The men were asked to ejaculate daily for seven days, but were not given any drugs or told to make any changes to lifestyle.
After seven days, their sperm was examined again.
The average of damaged sperm fell to 26 percent, placing it in the category of "fair" in quality.
Fourth-fifths of the men saw an increase in sperm quality, and many of them moved into the "good" range and out of the "poor" or "fair" categories.
However, one-fifth saw a decline in sperm quality.
Greening, an obstetrician and endrocrinologist at Sydney IVF, an Australian company that carries out assisted reproduction, said the improvements were "substantial and statistically highly significant."
Daily ejaculation not only boosted sperm quality for most of the men, it also helped sperm motility -- another big factor in successful fertilisation -- even though volumes of semen declined, he said.
Greening presented his findings at a conference in Amsterdam of the European Society of Human Reproduction and Embryology (ESHRE), which provided details in a press release.
The research did not investigate whether the improvement in sperm quality led to better pregnancy rates. But previous work has shown that sperm that is less damaged and more mobile has a better chance of leading to a healthy baby.
"These results may mean that men player a greater role in fertility than previously suspected, and that ejaculatory frequency is important for improving sperm quality," said Greening.
Why this is so is unclear.
Greening said he suspected that the longer sperm stays in the testicular ducts, the greater its exposure to rogue oxygen molecules that damage cells.
His advice to couples would be to have sex, or to ejaculate, daily in the runup to ovulation or to sperm donation for in-vitro fertilisation (IVF).
"The optimal number of days of ejaculation might be more or less seven days, but a week seems manageable and favourable," he said.
"It seems safe to conclude that couples with relatively normal semen parameters should have sex daily for up to a week before ovulation date."