The increasing participation in the athletic forms of bicycling warrants expanded physician attention to the traumatic and overuse injuries experienced by cyclists. The modern bicycle consists of a frame with various components, including handlebars, brakes, wheels, pedals, and gears, in various configurations for the various modes of cycling. For high performance cycling the proper fit of the bicycle is critical. The most efficient method to provide an accurate fit is the Fitkit, but proper frame selection and adjustment can be made by following simple guidelines for frame size, seat height, fore and aft saddle position, saddle angle, reach and handlebar height. The human body functions most effectively in a narrow range of pedal resistance to effort. Riding at too much pedal resistance is a major cause of overuse problems in cyclists. Overuse injuries are lower using lower gear ratios at a higher cadence. Cycling injuries account for 500,000 visits per year to emergency rooms in the US. Over half the accidents involve motor vehicles, and road surface and mechanical problems with the bicycle are also common causes of accidents. Head injuries are common in cyclists and account for most of the fatal accidents. Despite good evidence of their effectiveness, victims with head injuries have rarely worn helmets. Contusions, sprains and fractures may occur throughout the body, most commonly to the hand, wrist, lower arm, shoulder, ankle and lower leg. The handlebar and seat have been implicated in a wide variety of abdominal and genital injuries. Abrasions, lacerations and bruises of the skin are the most common traumatic injuries. Trauma may be prevented or reduced by proper protective safety equipment and keeping the bike in top mechanical condition. Anticipation of the errors of others and practising and adopting specific riding strategies also help to prevent traumatic injuries. Management of overuse injuries in cycling generally involves mechanical adjustment as well as medical management. Neck and back pain are extremely common in cyclists, occurring in up to 60% of riders. Ulnar neuropathy, characterised by tingling, numbness and weakness in the hands is common in serious cyclists after several days of riding. Managing saddle-related injuries or irritations may also involve adjusting seat height, angle and fore and aft position in addition to changing the saddle. Padding in the saddle and shorts play an important part in saddle problems. Saddle-related problems include chafing, perineal folliculitis and furuncles, subcutaneous perineal nodules, pudendal neuropathy, male impotence, traumatic urethritis and a variety of vulva trauma.(ABSTRACT TRUNCATED AT 400 WORDS)