The incident happened when a woman in her 40s was walking her dog in Ruislip Woods, near Wiltshire Lane at 6.40pm on July 21.
She was approached by a man wearing nothing but a white t-shirt, who then indecently exposed himself before leaving the scene.
The police have now released an e-fit of a man they wish to speak to in connection with the incident.
Detective Constable James Scarr, from the Western Area Local Investigations team, said: “This incident took place early on a Monday evening, not far from a residential area.
“Months have elapsed, and we have explored most lines of enquiry in this case – with limited forensic and CCTV opportunities available.
“We have taken the decision to release an e-fit of a man we wish to speak to in connection with the investigation.
“Do you recognise him?
“Or did you see anything out of the ordinary that day?
“Indecent exposure is a repulsive form of sexual offending, which can escalate to even more serious criminality.
“We are committed to stamping it out.”
They are asking anyone with information to call 101, quoting reference 6724/21JUL.
