ProTransport-1, the Cotati medical transportation company that was named one of the nation's fastest-growing companies by Inc. magazine in September, laid off about 5 percent of its workforce on Thursday.

In Sonoma County, 15 workers lost their jobs, and outside the county an additional 22 workers were let go. The affected employees were permanent workers, mostly in administrative roles, the company said.

"We are growing rapidly, but geared up our staffing faster than needed," Elena Whorton, president and co-founder, said in a statement. "That led to the situation where we have record business volume, but are over staffed and need to cut expenses."

The employees were let go without advanced warning and did not receive severance pay, said Carly Clements, public relations director for ProTransport-1.

"The decision was made, and we felt it best to let them know as soon as we knew," Clements said. "We have offered resources in securing them employment in other places, so we're really doing the best we can for them, but we weren't able to offer financial severance because we're cutting cost."

The company employs 723 people throughout Northern California, operating medical transport services for individuals and hospitals from Sacramento to Santa Clara.

"The result of the changes will be invisible to our patients and customers," Whorton said. "It's very difficult news to share, especially as we head into the holiday season."

ProTransport-1 started in 2000, when founders Whorton and CEO Mike Sechrist identified a need for a service to handle non-emergency medical transportation. Its annual revenues in 2011 were $39.4 million, up 72 percent over three years. In September, Sechrist said the company planned to triple its size and acquire more ambulance companies.

"They hired all these people, and boom, they're cutting jobs. I don't understand it," said Elroy Steele, whose wife, a collections agent, was let go. "We just spent all our money on Christmas . . . It's a really bad time."