Purpose Goeckerman therapy of psoriasis (GT) includes dermal exposure to crude coal tar ointment (CCT) and ultraviolet-radiation (UVR). The CCT contains polycyclic aromatic hydrocarbons (PAHs), some of them are immunosuppressive, genotoxic and carcinogenic. According to IARC both factors (CCT and UVR) are classified as human carcinogens. Combined exposure to PAHs and UVR can induce increase of mutations at chromosomal level (chromosome aberration) and creation of micro-nuclear structures and could increase the risk of malignant transformation of cells. This study describes the presence of chromosomal aberrations and various micronuclei structures in lymphocytes of psoriatic patients treated by dermal exposure to CCT and UVR. Methods The group of patients with psoriasis (n=32, male n=15, smokers n=13) was treated by GT (4% CCT applied topically and UVB whole body irradiation). The blood samples were collected before the first and immediately after the last procedure. We investigated (1) the number and distribution of micronuclei (MN), nuclear buds (gen amplification) and nucleoplasmic bridges (dicentric chromosome) in 1 000 binucleated lymphocytes (BNL) and (2) the structural aberration (SAL) and numerical aberration (NAL) in 100 metaphasical lymphocytes. Results Total number of BNL with MN (p<0.001), BNL with 1 MN (p<0.001), BNL with 2 MN (p<0.05) and BNL with GREATER-THAN OR EQUAL TO3 MN (p<0.01), total number of aberrated cells (SAL+NAL; p<0.001) and SAL (p<0.001) were significantly increased after GT. The smokers had significantly higher total number of aberrated cells (p<0.05) and number of smoked cigarettes correlated with total number of aberrated cells (rho=0.38, p<0.05). We also found correlation between age and BNL with MN (rho=0.55, p<0.001) and correlation between range of psoriatic lesions and total number of aberrated cells (rho=0.37, p<0.05). In conclusion, combined therapeutic exposure to CCT and UVR (Goeckerman therapy) increases the levels of the micronuclei and aberrations in lymphocytes of psoriatic patients and therefore increases the probability of malignant transformation of cells.