Hairy cell leukemia is often reported as a disease of young males. The male predominance is strong, 4:1, but the median age in the Swedish national compulsory cancer registry is similar to that of follicular lymphoma, i.e. 62 years. The overall 6-year survival in the Swedish registry of patients diagnosed since 2000 is 80%, 93% of patients <60 years, and 68% of those >60 years. The yearly risk of secondary cancers is 1.75%. Cladribine is a prodrug which is selectively activated intracellularly. The intracellular initial half-life is 13 h and the terminal half-life is 30 h. Subcutaneous injection once daily is simple and effective due to 100% bioavailability and no local side effects from injection, and self-administration is easy. Long-term follow-up of Scandinavian patients treated with cladribine (mostly as subcutaneous injections) in the early 1990s shows a >80% 15-year survival from cladribine treatment in <60 years of age, but <50% in older patients. Survival from diagnosis of these patients was similar for those previously treated and untreated.