The serum concentration of hyaluronan (HYA) was measured in 41 patients with malignant lymphoma, including 21 patients with non‐Hodgkin's malignant lymphoma and 20 patients with Hodgkin's disease. Thirty‐four patients were studied at diagnosis. The remaining 7 patients had relapsing or resistant disease. The patients were categorized into four stages according to conventional staging procedures. The median serum HYA concentration in patients with malignant lymphoma was significantly higher (median 40.7 ng/ml; 95% confidence limits 26.1–57.6 ng/ml) than in an age‐matched healthy reference group (median 14.5 ng/ml, 95% confidence limits 11–19.4 ng/ml) (P = 0.00032). The highest serum HYA levels were found in patients with relapsing/resistant disease, all being in stages III and IV (median 181.5; range 11.9–500 ng/ml), as compared to previously untreated patients (median 29.8; range 9.1–108) (P = 0.0002) and controls (median 14.2; range 6.7–51.2). Decreased uptake and degradation of HYA owing to malignant transformation of lymphatic tissue is the most plausible explanation to these findings.