Introduction: Onychomycosis and nail psoriasis can be embarrassing to patients, so improving the appearance of affected nails should be one of the key short-term goals of treatment. Methods: An 8-week open-label multicenter study was conducted to assess whether K101-03, a marketed topical treatment containing propylene glycol, glycerol, urea, and lactic acid, could produce rapid cosmetic improvements in affected nails. Adult patients with a big toenail or thumbnail (the “target” nail) affected by onychomycosis (n = 72) or psoriasis (n = 34) or both (n = 1) applied K101-03 to their affected nails once a day for 8 weeks. During and after treatment, patients rated the overall appearance of their target nail on a 4-point scale. They also assessed whether thickening, discoloration, brittleness, and softness of the target nail had improved since baseline. Adverse events (AEs) that occurred between the first application of K101-03 and the end of treatment were recorded and categorized according to severity and relationship to K101-03. Results: After 8 weeks of K101-03 treatment, 92.2% of patients (95% confidence interval [CI] 87.06-97.40) reported at least some improvement in the target nail. After 1 week of treatment, 78.3% of patients with onychomycosis (95% CI 68.53-87.99) reported at least some improvement in the target nail, and 55.1% of them reported that discoloration of their target nail had improved. Three patients reported a total of 5 AEs, none of which were judged to be related to K101-03. Conclusions: In summary, K101-03 was well tolerated in patients with onychomycosis or nail psoriasis and rapidly improved their nails.