The Stockholm-Gotland Cancer Register was used to evaluate the clinical observation that patients with non-Hodgkin's lymphoma (NHL) had an increased risk of malignant melanoma or squamous-cell carcinoma of the skin (SCCS) and vice versa. During 1958-1992, NHL was diagnosed in a total of 6,176 patients. Of these patients, 504 developed a second primary cancer of any type except NHL, compared to 301.9 expected, giving a standardized incidence ratio (SIR) of 1.7 [95% confidence interval (CI) 1.5-1.8]. The risk of SCCS and malignant melanoma in patients with NHL was 4.8 (95% CI 3.6-6.2; n = 54) and 1.7 (95% CI 0.8-3.1; n = 10), respectively. The hazard risk for a second malignancy was relatively constant over time, whereas the skin malignancies revealed the highest risks 3-10 years after initial diagnosis. Similarly, the risk of a secondary NHL was studied in patients with malignant melanoma and SCCS during the same period and found to be 1.3 (95% CI 0.8-2.1; n = 17) and 1.8 (95% CI 1.2-2.5; n = 34), respectively. The highest risk for NHL following malignant melanoma was seen 3-10 years after first diagnosis, while the highest risk following SCCS was observed 0-2 years after initial diagnosis. One of several possible explanations or the development of NHL and a skin malignancy in the same patient is an immunological defect caused by sun exposure.