Recent studies have proposed that exposure to ultraviolet radiation (UVR) protects against development of some internal cancers including that in prostate. This effect may be mediated by UVR-induced cutaneous synthesis of vitamin D. It is also proposed that ability to pigment in response to UVR will influence susceptibility to prostate cancer through its effects on vitamin D synthesis. We wished to determine first, whether ability to pigment, as assessed by skin type, influences the extent of exposure to UVR, secondly, whether skin type is associated with prostate cancer susceptibility and thirdly, whether such an effect is mediated by the extent of UVR exposure. We studied 453 prostatic adenocarcinoma and 312 benign prostatic hypertrophy (BPH) patients using a validated questionnaire to assess two parameters of exposure; months of cumulative exposure per year and adult sunbathing score. We used analysis of variance to show that cancer cases with sun-sensitive skin (skin type 1) had lower cumulative exposure per year (P = 0.014) and sunbathing scores (P < 0.0001) than those with type 4, possibly because of a tendency to avoid exposure. Further, cumulative exposure per year and sunbathing score were significantly lower in cancer compared with BPH patients (P < 0.001 and P < 0.001, respectively). While the proportion of subjects with skin types 1 and 2 was lower in cancer than BPH patients, these were not significantly different (logistic regression analysis, skin type 1 versus type 4; P = 0.11). We used recursive partitioning to determine if skin type influenced susceptibility to prostate cancer in subgroups stratified by exposure. Analysis of the data showed that in men with low sunbathing scores, skin type 1 conferred protection compared with skin types 2-4 (OR = 4.78, 95% CI 3.01-8.25, P < 0.0009). These findings indicate that susceptibility to prostate cancer is in part determined by extent of exposure to UVR and that ability to pigment mediates this effect.