In Puerto Rico, a shorter hair coat has been associated with a superior thermoregulatory capacity in slick (SLICK) Holstein cows relative to their wild type-haired (WT) counterparts. However, other hair properties may be associated with this tropical adaptation. For example, a thicker hair diameter has been reported as an adaptation of tropical Brazilian Holstein cattle. Thus, the aim of this study was to compare the hair diameter values between SLICK and WT Holstein cows in Puerto Rico. For this, 15 SLICK (2.77 ± 0.44 lactations; 595.05 ± 25.86 kg of body weight) and 19 WT (3.00 ± 0.44 lactations; 620.91 ± 21.44 kg of body weight) lactating Holstein cows from Lajas Agricultural Experiment Station (University of Puerto Rico) were compared. After local anesthesia infiltration, a skin biopsy (6 mm in diameter) was collected from the right shoulder of each cow. Skin samples were preserved in 10% formalin in histological cassettes for further analysis. Multiple photographs of each sample were taken perpendicular to the skin surface and compressed using a Canon EOS 7D Mark II camera and the Camlift V2.7.0 Zerene Stacker (64-bit) software. The measurement scale was set at 0.05 mm by Photoshop 6.0 (Adobe, San Jose, CA). The diameter at the base of 5 randomly selected hairs per biopsy was measured using the ImageJ software (v. 1.31). Measurements were replicated 5 times and averaged to obtain one value per skin sample (PROC MEANS). Data were analyzed by the GLIMMIX procedure in SAS. No differences in hair diameter were observed between SLICK and WT cows (0.0858 ± 0.0014 vs. 0.0838 ± 0.0014 mm, respectively; P = 0.6291). The observed diameter values in both hair coat groups coincide with those previously published by others in tropically adapted Brazilian Holstein cattle. Thus, it the evaluated WT cows might also have achieved considerable adaptations to tropical weather, avoiding any possible differences between phenotypes.