Paraffin sections from 21 cases of Hodgkin's disease (HD) and 28 cases (26 high-grade and 2 low-grade) of non-Hodgkin's lymphomas (NHL) occurring in childhood were examined for the presence of proliferating cell nuclear antigen using an anti-PCNA antibody. All cases of HD and NHL showed PCNA reactivity. In HD 50.9% (mean value) of Hodgkin and Reed-Sternberg (HRS) cells were PCNA positive and judged to be proliferating. PCNA reactivity was also found in a varying number of cells of the background population in HD (mean value = 11.7%). In NHL 61.2% (mean value) of cells were PCNA positive. In the 26 high grade tumours 63.6% (mean value) of cells were PCNA positive while only 32% (mean value) of cells were PCNA positive in the 2 low-grade tumours. Our results show that the proliferation rate of tumour cells in high-grade NHL is higher than those of tumour cells in low-grade NHL and HRS cells in HD. Moreover, we found a considerable variation of proliferation rate among individual cases of HD (range 31%-68%) of NHL (range 31%-78%). This suggests that PCNA assessment can help in the individual approach of the proliferation rate of each tumour, and, in conjunction with other parameters of the cell proliferation, could be useful for the understanding of the biological behavior of childhood lymphomas.