An oilspill risk analysis was conducted for the Gulf of Mexico Outer Continental Shelf (OCS)lease area region. Results of the analysis can be used to determine relative risks associated with oil production in different regions to be offered in OCS Lease Sales 72, 74, and 79. The analysis considered the probability of spill occurrences based on historical trends; likely movement of oil slicks based on a climatological model; and locations of major environmental resources which could be vulnerable to spilled oil. The times between spill occurrence and contact with resources were estimated to aid in estimating slick characteristics. Critical assumptions made for this particular analysis were (1) that oil exists in the lease area, and (2) that oil will be, found and produced from tracts sold in sales 72, 74, and 79. On the basis of a most likely resource estimate of 241 million barrels of oil to be produced over an 18-year production life from sales to be held in 1983 (sales 72, 74, 79), it was calculated that approximately one oilspill of 1,000 barrels or larger will occur. The estimated probability that one or more oilspills of 1,000 barrels or larger will occur and contact land after being at sea less than 30 days is 41-percent. For a high resource estimate case of sales to be held in 1983, 717 million barrels are estimated to be produced over an 18-year production life with an 83-percent chance of one or more spills of 1,000 barrels or larger occurring and contacting land within 30 days. These results depend upon the routes and methods chosen to transport oil from OCS platforms to shore. Given a total development scenario in which 5.6 billion barrels of oil are estimated to be present and produced, it was calculated that 18 oilspills of 1,000 barrels or larger will occur over the 40-year production life of the proposed lease area. The estimated probability that one or more oilspills of 1,000 barrels or larger will occur and contact land after being at sea less than 30 days is greater than 99.5 percent for this scenario. These probabilities also reflect the assumptions that oilspills remain intact for up to 30 days, do not weather, and are not cleaned up. It should be noted that the expected number of spills from the total development scenario is less than one-half that of the expected number from existing tanker transportation of crude oil imports in the Gulf area.