Background and Purpose. Hydrotherapy is popular with patients with rheumatoid arthritis (RA). Its efficacy as an aerobic conditioning aid is equivocal. Patients with RA have reduced muscle strength and may be unable to achieve a walking speed commensurate with an aerobic training effect because the resistance to movement increases with speed in water. The physiological effects of immersion may alter the heart rate–oxygen consumption relationship (HR–V˚O2 ) with the effect of rendering land-based exercise prescriptions inaccurate. The primary purpose of the present study was to compare the relationships between heart rate (HR), and ratings of perceived exertion (RPE), with speed during land and water treadmill walking in patients with RA. Method. The study design used a two-factor within-subjects model. Fifteen females with RA (47±8 SD years) completed three consecutive bouts of walking for five minutes at 2.5, 3.5 and 4.5 km/h–1 on land and water treadmills. Expired gas, collected via open-circuit spirometry, HR and RPE were measured. Results. HR and RPE increased on land and in water as speed increased. Below 3.5 km/h–1V˚O2 was significantly lower in water than on land (p<0.01). HR was lower (p<0.001), unchanged and higher (p<0.001) at 2.5, 3.5 and 4.5 km/h–1 in water than on land. RPE was significantly higher in water than on land (p<0.05). V˚O2 was approximately 60% of the predicted VO2 max during the fast walking speed in water. For a given V˚O2 , HR was approximately nine beats/min–1 and RPE 1–2 points on the 6–20 Borg scale, higher in water than on land. Conclusions. The study showed that the metabolic demand of walking at 4.5 km/h–1 was sufficient to stimulate an increase in aerobic capacity. The use of land based prescriptive norms would underestimate the metabolic cost in water. Therefore, in water HR should be increased by approximately 9 beats/min–1 to achieve similar energy demands to land treadmill walking.