Successes in cancer therapy have led to increasing numbers of cancer survivors, who are at risk of developing second primary cancers. Therapy- or disease-induced suppression of the immune function may predispose cancer patients to a second malignancy. An excess of squamous cell skin cancers (SCC) and non-Hodgkin's lymphomas has been found in immunosuppressed patients. We used the nationwide Swedish Family-Cancer Database on 10.2 million individuals to calculate the risk of second primary skin cancers and non-Hodgkin's lymphomas following a previous malignancy. A total of 4301 second skin cancers and 1672 non-Hodgkin's lymphomas were identified. Standardised incidence ratios (SIR)s and 95% Confidence Intervals (CIs) were calculated and compared. Among 14 different sites for male or female first primary malignancies, 11 of these sites were followed by an increased risk of skin cancer (SIRs for males for risk of skin cancer as a second primary cancer: 14.1 for SCC; 9.7 for melanoma; 6.1 for leukaemia as the first site; SIRs for females for risk of skin cancer: 14.6 for SCC; 6.8 for larynx; 6.2 for upper aerodigestive tract (UADT) as the first site). The risk of non-Hodgkin's lymphoma was increased after 10 of 14 different male neoplasms and 12 of 17 different female neoplasms. (SIRs for males for risk of non-Hodgkin's lymphoma as a second primary cancer: 6.4 for non-Hodgkin's lymphoma; 3.2 for leukaemias; 3.1 for multiple myeloma as the first site; SIRs for females for risk of non-Hodgkin's lymphoma as a second primary cancer: 12.5 for leukaemias; 7.0 for Hodgkin's disease; 3.6 for UADT as the first site). The high, and after certain sites, very high risks of second skin cancer and non-Hodgkin's lymphoma suggest that immune suppression may be a contributory mechanism.