Actinic Kerarosis (AK) are hyperqueratotic cutaneous lesions, in which an abnormal proliferation of keratinocytes of the epidermis is present [1]. Since 1960s, topical 5-Fluorouracil (5-FU), a hydrophilic antimetabolite drug, has been widely used as an effective treatment for many pre-cancerous conditions and certain malignant tumors. However, the severe inflammatory reaction, clinically characterized by erosion and ulceration, which occurs in the skin following topical therapy, is still a major treatment disadvantage. Topical delivery of hydrophilic drugs through intact skin usually creates problems due to the inability of the drugs to penetrate into stratum corneum. Liposomal formulations have demonstrated capacity to increase penetration across and into skin of these drugs when compared to conventional formulations [2]. Besides, when applied on the stripped skin, in absence of barrier for drug permeation, liposomes were found to provide targeted and sustained topical delivery [3]. Thus, we hypothesized that liposomal formulation containing entrapped 5-FU could be an interesting alternative for topical treatment of AK. 5-FU encapsulation efficiency (EE) and retention in liposomes are usually low (EE less than 10%). 5-FU does not associate with the lipid bilayer [4] and EE depends mainly on the internal aqueous volume of vesicles [5]. Therefore, EE has been greater in large unilamellar vesicles (LUV) when compared to multilamellar vesicles (MLV) and the physical state of the bilayer determines the retention capacity of the vesicles [6]. Recently, MLV liposomes containing 5-FU were prepared with high EE, but with an average vesicle diameter of 5 m [7].