OBJECTIVE Environmental and genetic factors influence thyrotoxicosis, but how population migration affects the disease panorama is not known. In an urban area in southern Sweden, during the years 1990 until 2003, the population increased from 232 000 to 267 000. In parallel, the proportion of inhabitants born outside Sweden increased. The aim was to investigate whether the incidence of Graves' thyrotoxicosis (GD), toxic multinodular goiter (TMNG), and solitary toxic adenoma (STA) has changed in Malmö since 1990 and to study the influence of geographic origin. METHODS An open label prospective registration of thyrotoxicosis, gender, age, smoking habits, and ethnic background during 2003-2005 in an urban area in southern Sweden (Malmö) was undertaken. RESULTS The total incidence of thyrotoxicosis has changed from 43 to 41.6 cases/100,000/year from the years 1988-1990 to 2003-2005 (p-value 0.72). GD increased from 22 to 29.6 (p-value 0.0051), TMNG decreased from 16 to 9.9 (p-value 0.0011) and STA from 4.8 to 2.1 (p-value 0.0054)/100,000/year. The total incidence of GD was higher in inhabitants born outside Sweden up to the age of 69 years and age-specific incidence showed a peak in women aged 50-59 years old in both groups. The greatest difference between immigrants and Swedes up to the age of 69 years was observed in women aged 20-29 years old. CONCLUSIONS The incidence of GD has increased and of TMNG has decreased in Malmö since 1990. Geographic origin seems to affect the incidence of GD. Whether the observed peak in age-related incidence in women 50-59 years is explained by menopausal changes has to be further investigated.