People with darker skin are about one-third more likely to have potentially dangerous pigment "spots" on their palms and soles.
In rare cases, these "acral pigmented lesions" turn out to be melanoma skin cancer. People with these lesions should have them checked by a dermatologist to be sure they are benign, the researchers said.
Reggae musician Bob Marley, for example, died from acral melanoma, which was diagnosed under his toenail.
"Acral pigmented lesions have not been well studied in people with darker skin," senior study author Dr Jennifer Stein, an associate professor in the department of dermatology at NYU Langone Medical Centre in New York City, said in a centre news release.
Stein's team evaluated the palms and soles of 1,052 patients seen at dermatology clinics in New York City and Miami. The researchers detected 391 acral pigmented lesions on palms and 278 on soles.
The lesions were found on 30 percent of white patients and 40 percent of those with darker skin – about a 30 percent difference.
The researchers also found that 44 percent of patients with the darkest skin had the lesions, compared with 28 percent of those with the lightest skin.
Acral pigmented lesions were also associated with a higher number of moles, especially among minority patients.
Only 54 percent of patients with these lesions on their palms and 43 percent who had them on their soles knew they had them, according to the study published online in the Journal of the American Academy of Dermatology.