OBJECTIVE--To describe the clinical assessment, diagnosis and differential diagnosis of dorsal vein thrombosis of the penis (DVTP) and to observe its natural course over time. DESIGN--A descriptive study of six patients presenting with penile swelling to an STD clinic over a twenty month period. SUBJECTS--Six male patients between the ages of 22 and 46 years who self-presented to an STD clinic in Perth, Western Australia during a period from October 1991 to June 1993. METHODS--Initial history, examination and follow up were undertaken as routine for all STD clinic patients. This was supplemented with later exhaustive history taking; full cardiovascular, fundoscopic, abdominal and genital examination; blood screening for coagulation defects, glucose level, autoantibodies, ESR, urea, electrolytes, calcium, creatinine and liver function test; and duplex doppler ultrasound scanning. RESULTS--No consistent abnormalities were detected on clinical examination, nor on blood testing. Ultrasound revealed one case of rupture of the corpus cavernosum, with haematoma and thrombus formation. There were two cases of pure DVTP demonstrable with ultrasound and two cases in which spontaneous resolution of clinical DVTP has occurred. The sixth patient declined further investigation and followup, but also displayed the clinical features of DVTP. Coagulation abnormalities as seen in elevated antithrombin III levels are of unknown significance. CONCLUSIONS--DVTP and ruptured corpus cavernosum should be considered in the differential diagnosis of gradual onset penile swelling and/or deformity. Its natural course tends to be one of spontaneous resolution. No sexual or urinary symptoms or dysfunction were experienced, even in the presence of persistent thrombus. Directed and specific investigation only, depending on the clinical state of the patient, should be carried out.