BACKGROUND While the majority of fistulas-in-ano are anatomically simple and easy to treat, a significant number are high or anatomically complex and have the potential to become a major management problem. METHODS One hundred and seven consecutive patients undergoing surgery for fistula-in-ano were studied prospectively with standardized anatomic diagrams. RESULTS Fistulas were classified as superficial (15%), intersphincteric (43%), trans-sphincteric (35%) or 'high' (7%). Within each group fistulas were considered either simple or complex (high tracks, extra tracks or other complications). Trans-sphincteric fistulas were more often complex than intersphincteric fistulas (32 vs 6%). A prior history of perianal sepsis and surgery was more frequent among the trans-sphincteric and 'high' groups. An external fistula opening within a narrow are 30 degrees either side of the posterior midline was almost always associated with a simple superficial or intersphincteric fistula (97%). Anterior and especially posterolaterally located external openings were frequently associated with complex fistulas (16 and 47%, respectively) and often had trans-sphincteric or 'high' tracks (58 and 56%). Goodsall's Law was more accurate for posterior (91%) and intersphincteric (93%) fistulas than for anterior (69%) and trans-sphincteric (68%) fistulas. Histopathology of fistula material showed unremarkable fistula-in-ano in 87% of requests. Six patients had unexpected abnormal results, including three new diagnoses of Crohn's disease. CONCLUSIONS The presence of additional anatomic complexity should always be anticipated in trans-sphincteric fistulas. Trans-sphincteric and 'high' fistulas are more likely to occur in females, and in patients with previous perianal sepsis or surgery for fistula. External openings close to the posterior midline almost always underlie simple fistulas, whereas posterolateral external openings are predictive of complex fistulas.