OBJECTIVE: to evaluate the effect of an immersion bath on pain magnitude during the first stage of labour. DESIGN: a randomised controlled trial comparing the pain scores of bathing and non-bathing nulliparous women during birth was employed. SETTING: the study was conducted at the Normal Birth Center of Amparo Maternal, São Paulo, Brazil. PARTICIPANTS: 108 birthing women, with 54 women randomly assigned to each group. INTERVENTIONS: when the birthing women presented at 6-7 cm of cervical dilation, they were placed in an immersion bath for 60 mins. OUTCOME MEASURES: pain scores, using a behavioural pain scale and a numeric scale, were recorded at two evaluation time points: at 6-7 cm of cervical dilation and 1h after the first pain score evaluation. FINDINGS: at the first evaluation, on the behavioural scale, the means were 2.1 for both groups (p=0.914; 95% confidence intervals (CI) 1.9-2.3 for the control group and 2.0-2.2 for the experimental group). On the numeric scale, the means were 8.7 and 8.5 for the control and experimental groups, respectively (p=0.235; 95% CI 8.2-9.2 for the control group and 8.1-8.9 for the experimental group). At the second evaluation, the pain score means for both scales were statistically higher in the control group than in the experimental group. On the behavioural scale, the scores were 2.4 vs. 1.9, respectively, for the control and experimental groups (p<0.001; 95% CI 2.2-2.6 for the control group and 1.7-2.1 for the experimental group). On the numeric scale, the scores were 9.3 vs. 8.5, respectively, for the control and experimental groups (p<0.05; 95% CI 8.9-9.7 for the control group and 8.1-8.9 for the experimental group). CONCLUSIONS: mean labour pain scores in the control group were significantly higher than those in the experimental group. The present findings suggest that use of an immersion bath is a suitable alternative form of pain relief for women during labour.