OBJECTIVE: To investigate the Ki-67 labelling index (LI) as a prognostic factor for the outcome of penile carcinoma, as in squamous cell carcinoma (SCC) of the larynx the expression of this marker correlates with histological features indicative of prognosis. PATIENTS AND METHODS: We retrospectively analysed the records of 44 patients in whom primary SCC of the penis was treated with amputation and bilateral lymphadenectomy (pT1, in 24, pT2 in 20, pN+ in 10; G1 in 12, G2 in 28 and G3 in four). During a mean follow-up of 35.6 months, four patients had disease progression. Tumour tissue was stained immunohistochemically using the streptavidin-biotin method. The mean Ki-67 LI was defined as the percentage of total tumour cells that were Ki-67-positive. The results were compared with pathological tumour stage, grade, nodal status and clinical disease progression. RESULTS: The mean (range) Ki-67 LI was 40.5 (6.4-93.0)%; a high mean Ki-67 LI was significantly inversely correlated with tumour differentiation (P < 0.005) and there was a tendency for a high Ki-67 LI to be associated with advanced local tumour stage, nodal metastasis and clinical disease progression, but these correlations were not statistically significant (P = 0.07, 0.07 and 0.06, respectively). CONCLUSIONS: The Ki-67 LI is correlated with tumour grade in penile cancer, and may indicate a greater risk of nodal metastasis.