Associations between hypertension and cardiovascular complications of diabetes mellitus in Nigerians, were examined in a cross-sectional study. 20 hypertensive-diabetic patients, 16 hypertensive patients, 10 non-hypertensive diabetic patients and 10 age- and sex-matched healthy controls, underwent M-mode and cross-sectional echocardiography, and Bruce-protocol treadmill exercise performance. Left ventricular (LV) mass indices (+/- SD) were significantly higher in hypertensive patients (164 +/- 12gm-2), diabetic (158 +/- 17gm-2) and hypertensive diabetic patients (125 +/- 129gm-2) compared with normal controls (111 +/- 17gm-2) p < 0.01. However, the LV mass index in the hypertensive-diabetic patients was significantly less than in hypertensive (p < 0.05) or normotensive diabetic patients (p < 0.05). Systolic cardiac contractility measured as fractional fibre shortening, was preserved in the hypertensive patients (24 +/- 4%) compared with the healthy controls (23 +/- 4%), but was depressed in diabetic patients (19 +/- 3%) and to a greater extent in the hypertensive-diabetic patients (15 +/- 4% p < 0.01). Treadmill exercise tolerance time was reduced independently in hypertension (309 +/- 73 seconds) or diabetes (321 +/- 119 seconds), p < 0.05, but was further impaired in hypertensive-diabetic patients (289 +/- 110 seconds) p < 0.01 compared to the healthy controls (490 +/- 156 seconds). The patients with hypertension and diabetes had a greater degree of proteinuria (p < 0.001) and a higher frequency of retinopathy (p < 0.001), in comparison to those with hypertension or diabetes alone.