Erythrocyte sedimentation rate (ESR), C-reactive protein (CRP) and serum lactate dehydrogenase (LDH) are non-specific biochemical markers, accompanying the growth of some neoplasms, both in adults and in children. The aim of this study was to determine the clinical value of ESR, CRP and LDH evaluation in the diagnosis, prognosis and monitoring of treatment among children suffering from cancer. 100 children patients with acute leukaemia, Hodgkin's and non-Hodgkin's lymphoma, nephroblastoma and soft tissue sarcoma were included in the study, being compared to 30 healthy children of the control group. In oncological patients all the markers were estimated prospectively at 5 stages or the disease, i.e.: before treatment, during treatment -- in partial remission (PR) and after complete clinical remission was achieved (CR), after therapy and during the relapse or progression of cancer (PROG). The mean pre-treatment levels of analysed markers in cancer patients were significantly higher than in healthy children (p<0.001). The elevation of ESR, CRP and LDH was observed in 78.7, 50.8 and 72.1% of cases respectively. Good clinical response to antitumour therapy was paralleled with the significant decrease of pre-treatment ESR, CRP and LDH levels, but the values observed in CR, both while treatment and after therapy, did not return towards normal range. The progression of disease was accompanied by the increase of ESR, CRP and LDH levels, however only ESR and CRP values differed significantly in PROG as compared to the CR phase. Among analysed markers, only LDH level at diagnosis proved to be an independent prognostic factor for the overall survival rate. Three-year overall survival rate for patients with pre-treatment LDH level <1.5 x normal value (N) was 94% while for those with LDH >1.5 x N -- 67%. It has been demonstrated that ESR, CRP and LDH determinations in children suffering from cancer may serve as useful markers both in diagnostics and monitoring of the disease course. Moreover, the pre-treatment LDH level appeared to have an important role in prognosis of the overall survival rate.