Objective. The effect of probiotics on IBS symptoms has been mixed, but remains an intriguing treatment option with appeal to the patient. Material and methods. Patients fulfilling the Rome II criteria were randomized double-blind to a daily intake of 500 ml of fermented milk containing at least 5 × 107 CFU/ml of Lactobacillus paracasei ssp paracasei F19, Lactobacillus acidophilus La5 and Bifidobacterium lactis Bb12 or an equal volume of acidified milk for 8 weeks. Symptoms were assessed at baseline and weekly using a disease-specific validated symptom rating scale (IBS-SSI). The predefined primary outcome measure was patient reported adequate symptom relief. Adherence to study protocol were assessed by performing stool samples at the of the treatment period. Results. Eight-one patients were screened. Sixty-four patients were randomized; 18 patients did not complete the study due to protocol violations or withdrew due to lack of effect. Fifty-two patients (13 males) completed the study as per protocol; mean age was 51.3 years (range 29-67). The proportion of patients reporting adequate symptom relief increased in both patient groups, but there was not any statistical difference between the groups. IBS-SSI scores did not differ statistically between the groups at the end of the treatment period, but improved during the study period in both groups. Conclusions. During this 8-week trial gastrointestinal symptoms improved. However, there was no difference between treatment with fermented milk containing probiotics or acidified milk. The effect of probiotics on IBS symptoms remains uncertain and further studies are warranted.