Over a 9-month period 228 men were admitted for acute urinary retention to six different casualty wards in the Copenhagen area. The patients were followed in the corresponding urological departments for one year. Twenty-seven variables were registered for each patient. The cumulative rate of recurrent retention was 56% after one week and 68% after one year. Factors predictive of preserved voiding ability were a retained volume less than 500 ml, a known event provocative of acute retention, and a maximum flow rate of more than 5 ml/s after the retention. The maximum flow rate measured within the first week after the retention was reliable within the follow-up period if the voided volume was 150 ml or more. The etiology of the acute retention was infravesical obstruction in 90% of the patients, and 85% required subsequent surgical treatment. Predictive of surgical treatment for infravesical obstruction were recurrent urinary retention within one week, a volume of retention of more than 500 ml, the absence of a provocative situation prior to the retention episode and nocturia twice or more.