Usually, lesions develop as a single, small plaque on the face of people aged 20-30 years with light coloring and significant sun exposure; they gradually progress in sun-exposed areas, such as the nose, the forehead, and the cheeks.

During periods of depressed immunity, the visible and the subclinical lesions flare and become inflammatory red and scaly. This depression of the immune system can occur following intense ultraviolet light exposure or with systemic chemotherapy for other carcinomas.

Over the years, the lesions gradually progress, and approximately 1 in 20 lesions eventually turns into invasive carcinomas. These lesions are typically the more red and inflamed, elevated, and indurated lesions. In very hyperkeratotic lesions, the invasive component of the carcinoma is at the base of the lesion and is often not seen until a physician removes the overlying plaque.

The morphology of atypical cells in both actinic keratosis and squamous cell carcinoma is identical. The risk of progression to squamous cell carcinoma (the most common form of skin cancer) is minimal. But up to 60% of squamous cell carcinoma cases begin as actinic keratosis. Regression of actinic keratosis occurs when sun exposure is decreased and mild lesions are healed.