In eighty consecutive patients who have the dysplastic nevus syndrome, the concentration of nevocytic nevi on the relatively sun-protected lateral thoracic area was compared to the concentration on the relatively sun-exposed areas of the anterior and posterior thorax. Nevocytic nevi in an area 7 X 20 cm were counted in each location. There was a total of 177 nevi on the lateral thorax (average, 2.2 nevi/person), 361 on the anterior thorax (average, 4.5 nevi/person), and 506 on the posterior thorax (average, 6.3 nevi/person). Men showed no significant difference in the number of nevi on the anterior and posterior thoracic areas, but women had fewer nevi on the anterior than on the posterior thoracic sites. These findings are consonant with the hypothesis that sunlight induces nevocytic nevi in patients who have the dysplastic nevus syndrome.