OBJECTIVE To evaluate the prognostic factors in uncomplicated venous leg ulcer healing. DESIGN Randomised parallel group controlled trial with subjects stratified by initial ulcer diameter and four months' maximum duration of follow up. SETTING Assessment at Northwick Park Hospital vascular unit with community based treatment. PATIENTS 200 patients with clinical and objective evidence of uncomplicated venous leg ulceration and an initial ulcer diameter > 2 cm. MAIN OUTCOME MEASURE Time to complete healing of the ulcer. RESULTS In the presence of graduated compression healing occurred more rapidly in patients with a smaller initial ulcer area (relative risk of healing 1.92 associated with halving of ulcer area (95% confidence interval 1.58 to 2.33)), shorter duration of ulceration (relative risk 1.35 associated with halving duration (1.17 to 1.56)), younger age (relative risk 1.34 associated with 10 year decrease (1.12 to 1.59)), and no deep vein involvement (relative risk 1.8 (1.19 to 2.78)). CONCLUSION These prognostic factors used in a simple scoring system predict time to healing.