Men making a lot of hands-free calls from a mobile phone stored in a pants pocket could damage their reproductive chances, a research group said.

Researchers at the Center for Reproductive Medicine at the Cleveland Clinic found that radio-frequency energy from mobile phones decreased sperm quality. The study of 361 men, posted on the clinic's Web site, found the greatest impact on those who used their phones for more than four hours a day.

While the initial study showed a correlation between semen quality and extended use of a mobile phone, researchers weren't absolutely sure of the association, and said more research was needed.

"The study results were sufficiently dramatic to attract extensive media attention, but additional research is necessary," researchers Ashok Agarwal and Edmund Sabanegh said in releasing the results of the study.

The researchers said electromagnetic waves from mobile phones could exert "deleterious effects" on sperm through thermal or nonthermal effects or a combination of both. "The exact mechanism is still unknown, and is one of the critical questions to be answered."

The center's research is not the first to find harmful effects from mobile-phone use. In July, the director of the University of Pittsburgh Cancer Institute urged faculty and staff to limit the use of mobile phones because of possible cancer risk.

While acknowledging that the evidence thus far was controversial, Dr. Ronald B. Herberman said he was convinced "that there are sufficient data to warrant issuing an advisory to share some precautionary advice on cell phone use."

Earlier this year, a medical journal said studies showed that heavy users of mobile phones face a 50% greater risk of developing cancer in their parotid gland, located near the jaw and ear, which is where cell phones are commonly held. However, Danish scientists studying 420,000 mobile phone users concluded that the devices don't cause cancer. Herberman based his concerns on unpublished data.