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WASHINGTON (CNN) - When states eliminate laws requiring motorcycle riders to wear helmets, the likely result is a 50 to 100 percent increase in motorcycle fatalities, according to a report released Monday.

The study focused on Florida, which repealed its helmet law in 2000, but looked at several other states, as well.

While several factors make such an outcome likely, a major reason is the dramatic decline in helmet use, "from virtually full daytime compliance to voluntary use by about 50 percent of riders," said the report, released by the National Highway Traffic Safety Administration.

Since 1995, when Congress dropped federal sanctions against states that didn't require helmets, several states have removed laws that required all motorcycle riders to wear them.

At the end of 2003, 28 states required helmet use only by riders under a specific age or for other limited groups, and three states had no requirements whatsoever, the report said.

Florida's law requires helmet use for riders under age 21 and those without a set level of insurance, but helmet use fell among those groups as well because such a selective requirement is difficult to enforce, the report said.

In 2001 through 2002, 45 percent of riders under age 21 who were killed were not wearing helmets, the report said, compared with 26 percent in the two years before the law changed.

An increase in motorcycle registrations also contributed to the increase in deaths, along with an increase in the number of inexperienced riders, the report said. Motorcycle registration rates were about 50 percent higher in states without universal helmet laws compared with states with them, it said.

Florida's motorcycle deaths increased by 81 percent, it said, well above what would have been expected with the increase in motorcycle registrations alone.

The report also listed numbers from four other states -- Arkansas, Kentucky, Louisiana and Texas -- that eliminated all-rider helmet requirements. It noted increases in fatalities ranging from 23 percent to 130 percent in those states.

Among non-helmeted riders, fatalities in Florida increased from 15 in 1998 to 198 in 2002, the report said. Fatalities of non-helmeted riders made up two-thirds of the total motorcycle deaths in Florida in 2002.

Adjusted for inflation, medical costs for motorcyclists injured in crashes more than doubled in Florida, from $21 million to $44 million for the years before and after the law change, respectively, the study said, and the average cost per case rose from about $34,500 before the change to about $39,900 after.

Nationwide, fatalities in motorcycle crashes have increased for seven consecutive years, with 4,008 killed in 2004, an 8 percent increase over 2003, according to NHTSA.

Louisiana returned to an all-rider helmet law on August 15, 2004, after experiencing fatality results similar to those found in the Florida study. Michigan is considering changing its all-rider law to an under-21 law.

The study was conducted by the Preusser Research Group for NHTSA.

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