The aim of this study was to investigate whether uptake of carbon-11 methionine (MET) is associated with histological grade of malignancy and survival in patients with newly diagnosed or recurrent lymphoma. Thirty-two patients with histologically confirmed lymphoma participated in the study. Twenty-six (81%) were studied before any therapy and six before treatment for recurrent disease. Twenty-eight patients had non-Hodgkin's lymphoma and four had Hodgkin's disease. An ECAT 931/08-12 positron emission tomography (PET) scanner was used for PET imaging. After the transmission scan, a median dose of 293 MBq of MET was injected intravenously and dynamic images were acquired for 40 min. The uptake of MET in tumour was measured as the standardized uptake value (SUV) and influx constant (Ki). The SUV formula was also adjusted to the predicted value of lean body mass (SUVlean) and body surface area (SUVBSA). The PET results were correlated with the clinical follow-up data. The median SUV in 32 malignant lesions was 6.6 (range, 1.9-12.4) and the median Ki was 0. 116 min-1 (range, 0.025-0.201, n=23). The median SUV was 7.0 (range, 5.4-12.4, n=9) in high, 6.2 (range, 1.9-10.4, n=11) in intermediate and 5.7 (range, 3.8-8.3, n=8) in low grade lymphomas. One intermediate grade lymphoma of the skin was visually negative (SUV 1. 9). In Hodgkin's disease the median SUV was 7.0 (range, 3.2-7.9, n=4). The median Ki value was 0.162 min-1 (range, 0.147-0.197, n=7) in high, 0.099 (range, 0.025-0.152, n=10) in intermediate, and 0.078 (range, 0.056-0.152, n=4) in low grade lymphomas and 0.149 (range, 0. 096-0.201, n=2) in Hodgkin's disease. The difference between high and other grade non-Hodgkin's lymphomas was significant when using Ki (P<0.001), but not with SUV, SUVlean or SUVBSA. The final outcome of the patients was not related to MET uptake. Lymphomas with a high Ki value tended to have a high S-phase fraction (r2=0.46, P=0.043). It is concluded that MET PET is highly sensitive for the detection of untreated and recurrent lymphomas. Differentation of high grade lymphomas from lower malignancy grades seems to be possible if graphical analysis is applied to calculate Ki for MET. However, prediction of survival is not possible with MET PET.