Blood, plasma and serum viscosity and blood filtration were investigated in 43 unselected mixed cryoglobulinemia patients. A hyperviscosity syndrome was present in only one patient, and rapidly improved after plasma exchange and cyclophosphamide therapy. A clear-cut increase in blood viscosity was infrequently observed in mixed cryoglobulinemia, although significant differences were present in the plasma and serum viscosity of patients and controls. In contrast, blood filtration was severely impaired in a high percentage of cases (51 and 72% of the values recorded at 37 degrees and 25 degrees C, respectively), and was on the average significantly higher in patients than in controls. Indirect evidence suggests that blood viscosity is at least in part related to cryoglobulins. In 19 patients studied before and after cryoglobulin removal, serum viscosity significantly decreased when the serum was deprived of cryoglobulins. In addition, the cryocrit correlated with all the hemorheological parameters with the exception of blood filtration. The hemorheological findings were compared with multisystemic features of the disease, i.e. liver, renal, lung, neurologic, vascular and funduscopic alterations. The potential clinical relevance of the hemorheological parameters was stressed by the close correlation between blood filtration parameters and serum creatinine. Furthermore, by discriminant analysis, viscosity and blood filtration changes were the serological parameters most significantly associated with the presence of renal, liver and neurological involvement. Thus, hemorheological parameters are frequently abnormal in mixed cryoglobulinemia patients, and seem to play a significant clinical role; they should therefore be included in the work-up of these patients.