OBJECTIVE: To examine the time of occurrence, type and cost of injuries resulting from bicycle accidents treated in Emergency Departments (ED). METHODS: Cases were identified from 2002 ED and hospital databases from Massachusetts using ICD-9 diagnosis and E-codes (E826.0-E826.9). Hospital cases were limited to those admitted via the ED. Cost estimates include facility, accommodations and ancillary services, reported in 2002 US$. Charges were adjusted using a 0.61 cost-to-charge ratio. RESULTS: Of 10,025 bicycle accident cases identified, 75% were male. Mean age was 28 years (median: 15, range: <1-98), 61% were under 20 years. Transport by ambulance was used for 13% of those injured; <1% by helicopter. The cyclist was the injured party in 96% of cases; a pedestrian or other person in 4%. Almost half (48%) arrived between 4:00 and 9:00 PM, 36% on a weekend and 70% during May-September. The majority (94%) were treated and released from ED, 2% died in ED, 3% were admitted to hospital; and 1% were either transferred to another facility, or left AMA. Highest admission rate (12%) was seen in older patients (60+years). Skull fractures and other head injuries accounted for 6% of cases. Other fractures, dislocations and sprains represented 37%, 27% were mainly contusions/abrasions, 23% had open wounds, and <1% major abdominal injury. The visit reason codes for the remaining cases included pain, other symptoms and conditions. Mean duration of ED visit was 2.3 hours (median: 1.8). Mean cost per ED visit was $460 (median: $325) for those treated and released and $560 for those admitted (median: $427). On average, those admitted spent three days (median: 2) in the hospital at a cost of $8375 (median: $6340). CONCLUSIONS: Most bicycle related injuries occurred in children or young adults,in the early evening, involved only the cyclist and could be managed in the ED.