The purpose of this study was to investigate frequency and pattern of urological abnormalities in children younger than two years of age in the county of Frederiksborg, Denmark, with a catchment area of about 350,000 people, and furthermore to investigate in which way the urological abnormalities were diagnosed. From 1994-1996 a total of 35 children younger than two years of age were found to have urological abnormalities. Fourteen patients had a ultrasonographical prenatally diagnosed urological abnormality, which was confirmed postnatally. Twenty-one children were diagnosed with urological abnormalities after presentry with their first urinary tract infection. The obstructive urological diagnoses: congenital hydronephrosis, congenital megaureter and posterior urethral valves made up to 57% (20/35) of the abnormalities. Nine boys were operated upon; three for unilateral hydronephrosis, two for posterior urethral valves, one for unilateral megaureter and three for phimosis. Two girls were treated with oestrogen for labial fusion. Urological abnormalities which were treated were found among the ultrasonographically diagnosed abnormalities, apart from phimosis and labial fusion. The frequency of urological abnormalities was 0.25% in children younger than two years of age in Frederiksborg county. In conclusion, in 1994-1996 the frequency and pattern of urological abnormalities in Frederiksborg county was in accordance with the figures in the literature. Urological abnormalities requiring treatment were diagnosed in 11 children; 55% (6/11) ultrasonographically and 45% (5/11) by examination of the external genitalia of children younger than two years of age at the time of the first diagnosed urinary tract infection.