Background: This is an observational study with a proprietary extract of willow bark (Assalix®) under conditions of daily practice in Switzerland. The scope of the study was to get a better estimate of the frequency of adverse events (and possibly identify unknown adverse reactions) and a broader picture of the efficacy. Methods: Any adult patient eligible for treatment with the proprietary extract of willow bark could be admitted to the trial. The study had a duration of 6–8 weeks, with an intermediate control visit after 3–4 weeks. Besides the customary demographic and anamnestic data, the variables assessed (intent to treat) were pain intensity, impairment of daily activities and global assessment of efficacy and tolerability. Results: 204 participating physicians treated 877 patients with different types of rheumatologic pain (Females 64.2%, age 58.76 ± 15.69 years); 763 patients completed the study. The medical problem had persisted more than six months in 68% of the cases and 81.2% of the patients had already received another treatment. Additional anti-inflammatory drugs were co-prescribed in 39.3% of cases. The pain score at admission was 5.32 ± 1.62 (on a scale of 0–9) and at last visit it was 2.51 ± 2.04 (p<0.001); ‘total pain relief’ was reported by 14% of patients. While at admission 0.6% of the patients reported no impairment of daily activities, at last visit the proportion was 27.4%. Thirty-eight patients (4.3%) reported a total of 46 adverse events relating predominantly to digestive system (3.1%) and skin (1.6%). There were no ‘serious’ clinical adverse events. Adverse events were more frequent in cases receiving additional anti-inflammatory medication. Conclusions: The proprietary extract of willow bark (Assalix®) was well tolerated, with no unexpected adverse events identified. With the limitations inherent to the study design, it may be concluded that it was moderately effective as an analgesic in the management of dorsopathies, soft tissue disorders, inflammatory polyarthropathies and arthrosis.