We made a prospective study of 116 patients with tibial diaphyseal fractures who had continuous monitoring of anterior compartment pressure for 24 hours. Three patients had acute compartment syndrome (2.6%). In the first 12 hours of monitoring, 53 patients had absolute pressures over 30 mmHg and 30 had pressures over 40 mmHg, with four higher than 50 mmHg. Only one patient had a differential pressure (diastolic minus compartment pressure) of less than 30 mmHg; he had a fasciotomy. In the second 12-hour period 28 patients had absolute pressures over 30 mmHg and seven over 40 mmHg. Only two had differential pressures of less than 30 mmHg; they had fasciotomies. None of our 116 patients had any sequelae of the compartment syndrome at their latest review at least six months after injury. A threshold for decompression of 30 mmHg would have indicated that 50 patients (43%) would have required fasciotomy, and at a 40 mmHg threshold 27 (23%) would have been considered for an unnecessary fasciotomy. In our series, the use of a differential pressure of 30 mmHg as a threshold for fasciotomy led to no missed cases of acute compartment syndrome. We recommended that decompression should be performed if the differential pressure level drops to under 30 mmHg.