Midkine (MK) is a member of a family of heparin-binding growth factors, which are reported to be angiogenic. We have investigated by RNase protection analysis the expression of MK in 47 primary bladder tumors and 7 normal bladder samples. MK mRNA transcripts were detectable in 46 (98%) of 47 of the tumors and in 5 (70%) of 7 of the normal bladder samples. However, median MK expression was 4-fold higher in tumors than in the normal bladder (P < 0.004). In eight tumors (17%), MK expression was elevated more than 10-fold compared with the median value of the normal bladder specimens. There was no statistically significant difference in expression between superficial and invasive tumors (P < 0.50). Seven (32 %) of 22 patients with invasive cancers are alive at 1 year with no evidence of recurrence; in 5 (70%) of these patients, MK expression in the tumor was within the normal range at the time of presentation. By contrast, in only 2 (13%) of 15 patients who died or whose tumors recurred or progressed was MK expression in the normal range (P < 0.01). Overall, median MK expression in invasive tumors that caused death, progressed, or recurred within 12 months was 3-fold higher than that found in the tumors of those patients who were clear of disease at 12 months (P < 0.04). Thus, overexpression of MK is associated with the development of bladder cancer and in invasive cancers predicts a poor clinical outcome in the short term.