Objective: To describe the epidemiology and risk factors for primary episodes of severe lower leg cellulitis (LLC). Methods: This was a longitudinal cohort study using state-wide data linkage of adults presenting to Western Australian (WA) hospitals with a first ever LLC from January 2002 to December 2013. The study aimed at determining risk factors, medical records from the index patient, together with comparable data from controls matched by age, sex, postcode, and month of admission. Results: During the period, 36 276 patients presented with their first episode of LLC. The incidence increased by 4.7% per annum, reaching 204.8 (95% CI 198.6–211.1) per 100 000 population by December 2013. Analysis of 29 062 case-control pairs showed several conditions with lower limb pathology were independently associated with LLC, including varicose veins (AOR 2.95, 95% CI 2.50–3.48, p <0.001), lymphoedema (AOR 2.65, 95% CI 1.71–4.10, p <0.001), tinea pedis (AOR 3.05, 95% CI 1.45–6.42, p 0.003), and saphenous vein harvest during coronary artery bypass grafting (AOR 1.74, 95% CI 1.32–2.30, p <0.001). Also associated with LLC was obesity (AOR 2.05, 95% CI 1.82–2.31, p <0.001), renal disease (AOR 1.28, 95% CI 1.14–1.44, p <0.001), rheumatologic conditions (AOR 2.12, 95% CI 1.72–2.60, p <0.001), hemiplegia/paraplegia (AOR 1.31, 95% CI 1.13–1.52, p <0.001), and liver disease (AOR 1.77, 95% CI 1.51–2.06, p <0.001). Conclusions: LLC presents a major burden to the health sector and is increasing with an ageing population. Given the high rates of recurrence, long-term morbidity, and economic impact, efforts to reduce primary episodes should be incorporated into the infectious diseases and healthy ageing research agenda.